Men's Knitwear
Most men's knitwear is bought to go over something else, which makes layering room the fit question that actually decides the size — not the chest measurement on its own. A sweater sized close over a t-shirt will pull across the back and shoulders once a collared shirt is underneath it, and brands differ substantially in how much room they build into a given chest size for exactly that reason. The product's own fit description ("slim", "regular", "relaxed") predicts this better than the size letter does.
Body length is the measurement most often ignored and most often wrong. A sweater worn over a shirt needs to reach past the shirt's hem, or the shirt tail shows below it every time you move; a cropped or boxy modern cut is deliberately shorter than that and is a style choice, not a sizing error to correct by going up a size. Where a retailer publishes a back-length measurement, comparing it against a sweater you already own tells you more than the size label will.
Neck style changes both fit and how much of the shirt underneath stays visible: a crew neck covers the collar entirely, a half-zip or a v-neck frames it, and a roll neck rules out wearing a collared shirt under it at all. Fibre matters for how long any of this lasts — wool and wool blends recover their shape after stretching, while a pure cotton knit tends to grow at the elbows and hem across a season and stay grown.
Men's Knitwear at retailers

Banana Republic Factory
Merino Wool Sweater

COS
Basketweave Wool Sweater

COS
Ladder-Stitch Wool Sweater
asos
ASOS DESIGN oversized knit crew neck wool sweater in cream
asos
ASOS COLLECTIVE oversized knit wool sweater in ecru

Banana Republic Factory
Merino Wool Crew-Neck Sweater

Banana Republic Factory
Merino Wool Quarter-Zip Sweater

Banana Republic Factory
Merino Wool Polo Sweater

Banana Republic Factory
Merino Wool V-Neck Sweater

COS
Boiled-Wool Crew-Neck Sweater
Calvin Klein
Calvin Klein Men's Merino Wool Classic Sweater - Black - XL

COS
Ribbed Wool Mock-Neck Sweater
Calvin Klein
Calvin Klein Men's Merino Wool Relaxed Sweater - Green - XS
Calvin Klein
Calvin Klein Men's Merino Wool Classic Sweater - Green - XS

Banana Republic Factory
Merino Wool Textured Sweater

COS
Ribbed Wool Crew-Neck Sweater

COS
Slim Merino Wool Crew-Neck Sweater

COS
Pocket-Detail Wool Crew-Neck Sweater

COS
Striped Boiled Wool V-Neck Sweater
asos
ASOS DESIGN oversized knitted crew neck wool blend sweater in sage