How to Find Your Size in Oversized Cotton Shirts

By Ella Blake, Tellar Stylist

For an oversized cotton shirt, you want to size up one or two sizes from your usual — but here's the bit nobody tells you: the shoulder seam still needs to sit close to your actual shoulder, or the whole thing tips from deliberately relaxed into genuinely too big. I learnt this the hard way after buying an oversized shirt in what I thought was a charmingly roomy size large, only to realise the shoulder seam was sitting halfway down my arm and I looked like I'd raided my dad's wardrobe circa 1987. Not the vibe. The good news is that once you know your shoulder measurement and understand how different brands approach "oversized," you can nail this every single time.

Why the Shoulder Seam Is Everything

I say this with all the authority of someone who has returned a frankly embarrassing number of shirts: the shoulder seam is the non-negotiable fit point on any shirt, oversized or otherwise. Everything else — the body, the sleeves, the length — can be worn loose, rolled up, tucked in, or belted. But if the shoulder seam is drooping down your arm, nothing will save the outfit.

Here's how to measure your shoulder width properly:

Ask someone to help, or stand in front of a mirror
Measure straight across your upper back, from the edge of one shoulder to the other — the bony tip to bony tip
This is your shoulder measurement. Write it down and keep it in your phone

When you're buying an oversized shirt, you want the shoulder seam to sit right at or just slightly off the edge of your actual shoulder — perhaps 1–2cm off the edge at most. Any more than that and it stops looking intentional.

How Much to Size Up (and When to Stop)

The beauty of an oversized cotton shirt is the relaxed, thrown-on ease of it. But there's a spectrum, and knowing where you want to land on that spectrum before you shop makes the whole process easier.

One size up: Gives you a slightly relaxed, boyfriend-shirt feel — a little room in the body and sleeves but still looks put-together. Great for tucking into tailored trousers or wearing half-tucked with jeans.
Two sizes up: A proper oversized look — boxy body, roomy sleeves, longer length. Works brilliantly belted over wide-leg jeans or as a beach cover-up over a swimsuit. This is the sweet spot for most people going for that Scandi-casual aesthetic.
Three or more sizes up: Now you're in dress territory, which can look incredible — shirt dress, anyone? — but needs intentional styling. Wear with cycling shorts or bike shorts beneath, or belt it properly and let it be the whole outfit.

Cotton Shirts and Shrinkage: Size for the Wash

This is a genuine oversized-shirt-specific issue that catches people out constantly. Pure cotton shrinks — sometimes significantly — on the first wash, and if you've bought something that's only slightly oversized, it can become fitted after one spin in the machine. My rule? If a brand doesn't specify pre-washed or pre-shrunk cotton, size up an extra half size mentally when you order.

Fat Face — brilliant for relaxed, quality cotton casualwear — uses a lot of 100% cotton that benefits from being washed on a cooler setting initially. Seasalt Cornwall, another fantastic brand for easy cotton shirts, tends to pre-wash their fabrics, which means what you see is what you get, wash after wash. Worth checking the product description before you buy.

If a shirt is a cotton-poplin blend or includes a small percentage of elastane, it's far more wash-stable and will keep its shape much better over time. Worth bearing in mind if longevity matters to you.

High Street Brands — and How They Size Their Oversized Shirts

Not all oversized shirts are created equal. Here's what I genuinely think of the main players based on customer reviews, styling press, and personal experience:

The Gap: Consistently excellent for oversized cotton shirts. Their classic Oxford shirts in particular have been a staple for decades — generously cut, great quality cotton, and they wash beautifully. Their sizing is honest and reliable, which makes ordering online much less stressful.
Abercrombie & Fitch: Has had a serious glow-up in recent years and their oversized shirts are genuinely lovely — quality cotton, well-finished, and the sizing is consistent. Their "boyfriend shirt" styles are especially popular and tend to get brilliant reviews for fit.
Mango: A go-to for oversized shirts with a slightly fashion-forward edge. They do great linen-cotton blends too. Runs slightly small for a European brand, so size up one from your usual.
Cos: Masters of the oversized, boxy shirt. If you want clean lines and a minimal, architectural feel, nobody does it better. Their cotton quality is excellent and sizing is consistent — though they do run narrow in the shoulder, which is worth knowing.
Barbour: Unexpected, perhaps, but their cotton shirts are beautifully made and have a lovely relaxed fit without feeling shapeless. A brilliant choice if you want something that looks effortlessly put-together rather than trend-led.
Hush: One of the most underrated brands on the high street for exactly this. Their oversized cotton shirts are a perennial bestseller — soft, well-cut, and available in great colours. Sizing is generous and consistent.
Whistles: Their oversized shirts have a slightly more premium, elevated feel without the designer price tag. Great for taking an oversized shirt into smarter territory — think tucked into wide-leg tailored trousers for the office.
Crew Clothing: Solid, dependable, and honestly very underappreciated. Their cotton shirts are excellent quality and their oversized styles wash well and keep their shape. Good sizing consistency too.

Premium and Designer Picks Worth the Splurge

If you're investing more, the quality really does show in cotton shirts. Massimo Dutti does exceptional quality cotton shirts with a polished, considered cut — their oversized styles feel luxurious without being ostentatious. For something truly special, Max Mara does the oversized shirt in that very Italian, grown-up way that just looks expensive from across the room. Their cotton quality is outstanding and the cut is flattering in a way that cheaper versions rarely manage.

Two Independent Brands I'm Currently Obsessed With

Deiji Studios: An Australian independent brand that has built a serious cult following for their cotton shirts and sets. The fabric is extraordinary — incredibly soft, pre-washed, and the fit is that perfect "I borrowed this from someone cool" oversized without ever looking sloppy. Sizing notes on their website are detailed and accurate.
Asceno: A London-based independent brand making some of the most beautiful cotton and linen shirts I've come across. Their oversized styles have a slightly luxurious, laid-back quality that punches well above the price point. Worth every penny if you want something that'll last years rather than one season.

Styling an Oversized Cotton Shirt: The Cheat Sheet

Once you've got the fit right, the styling possibilities are genuinely endless. A few of my favourite ways to wear one:

Half-tucked into straight-leg jeans — the easiest, most reliably stylish combination. Works with white trainers or loafers equally well
Fully open as a layer over a fitted vest or ribbed tank and wide-leg trousers — very current, very easy
Belted at the waist to create shape and turn an oversized shirt into a mini shirt dress — brilliant for summer with sandals
Knotted at the hem over high-waisted denim or tailored shorts — that classic relaxed-but-intentional look
As a beach cover-up over a swimsuit — an oversized white cotton shirt over a bikini is one of those eternally perfect combinations

A Quick Note on Sleeve Length

Most oversized shirt styling involves rolling the sleeves — which neatly sidesteps the sleeve length issue for shorter arms. But if you're tall or have longer arms and hate having to roll, it's worth checking the sleeve length measurement in the size guide before you buy. Many brands don't list this, which is frustrating, but the ones that do — Gap and Abercrombie & Fitch especially — make ordering so much easier.

Find Your Exact Size in Oversized Shirts — Instantly

Oversized sizing is genuinely confusing because "relaxed" means something different across every brand. That's exactly why Tellar exists. It's the UK's leading free sizing tool — enter your measurements once (bust, waist, hip, or an existing brand size you trust) and Tellar instantly tells you your correct size across 1,500+ brands. No more size guides, no more guessing, no more disappointing returns.

Use the Store Size Lookup tool to check your size in any brand instantly — whether that's Cos, Gap, Mango, Whistles or hundreds more. It's completely free and works straight in your browser — nothing to download, no sign-up faff.

And while you're exploring, head to the Tellar Fashion Hub for a growing library of honest, independent style guides written by real stylists. No sponsored content, no brand bias — just genuinely useful fashion advice, always free.