You’d think this would be fairly easy, wouldn’t you? Put on shirt. See if shirt feels comfortable. If it does, buy it. But most men’s shirts, particularly in the US, are made larger than a man needs. Because everyone notices when he tries on something that’s too tight. Far fewer notice if it’s too big.
Let’s start with the sleeves. While there is an element of personal taste involved in the length of a jacket’s sleeves, the rule for a shirt’s sleeves is pretty universal. The end of the cuff should finish at the base of your thumb (when your arms are by your side). It should grip your wrist fairly tightly, so that there can be some excess in the length of the sleeve, but it does not fall down to envelope your hand. This excess in the length means that when you stretch your arm out, the cuff remains on your wrist and is not tugged back.
Next the collar. Again, most men wear collars that are too big. Because no one minds a little breathing room; but everyone hates being strangled. There is certainly some room for personal taste, but don’t stray too far from the rule: when it is fastened you should be able to easily slip two fingers between the collar and your neck. Three is ok. Four or five means it is too big. Your tie will droop, your shirt will look apologetic and you’ll look like you’re wearing your big brother’s clothes.
The fit of a shirt’s body is much more subjective. Some people prioritise comfort above a sharp silhouette. Just as many go too far the other way. But let’s start with a certainty: the longer the tail of your shirt, the better it will fit. Because regardless of how big the shirt is around your waist, it will be baggier if the tail is short. A long tail creates greater friction between shirt and trousers, and curves around your legs, making it far harder to pull out. So the shirt bags less and looks better. The only problem is if you like wearing the shirt untucked as well. Then you have to compromise.
On the body of the shirt, there are three measurements the manufacturer will have considered: chest, waist and hips. Your natural waist is around your belly button. Your hips are probably where your trousers start. Hips may seem the least important of the three, but the width here makes almost as much difference to bagginess as the length of the tail. Keep it narrow if you can. As to waist, make sure it looks good to your eye but has room to stretch when you sit down (everyone’s waist expands then, no matter how slim). And with the chest, reach around behind you and make sure the buttons don’t pop – oh, and you want as high an armhole as possible as it will allow greater manoeuvrability.
Those are the main points to watch when you try on a new shirt, stare into the mirror and look yourself up and down. You’ll never find one that ticks them all unless you have it made to measure. But you will find one that’s close if you bear these points in mind and invest some time tramping between stores. And once you’ve found a brand that fits well, stick to it.