THE FIRST PRINCIPLE
Fit is the difference between “nice shirt” and “you look good.”
Price is invisible. Fit is not. Before buying anything else, learn the five places clothes tell on you.
Shoulders
The seam should end where your shoulder ends. Too far down and the shirt looks borrowed. Too far in and the fabric pulls across your upper back. Shoulders are the hardest and most expensive thing to alter, so get them right at purchase.
Sleeves
A dress shirt cuff should land at the wrist bone and show roughly a quarter to half inch below a jacket sleeve. Casual sleeves can break the rule, but they should never swallow your hand.
Torso
You want clean lines, not vacuum sealing. Pinch two to three inches of fabric at the side of a shirt. Much more usually looks sloppy; much less usually looks strained when you sit.
Trousers
The waistband should stay in place without a belt. The seat should lie clean, and the front pockets should not flare. A slight break at the shoe is the easiest modern default.
Jacket length
A classic jacket covers most of the seat and visually divides your body in half. Fashion crops come and go. Balanced proportions keep working.