A blazer is the garment where fit is most visible and most misunderstood. Work through these in order, because they are ranked by how difficult they are to change.
1. The shoulder — cannot be altered
The seam where the sleeve meets the body should sit at the bony point of your shoulder. Not past it, not inside it.
Too wide and the sleeve head collapses into a divot below the seam. Too narrow and a diagonal pull appears from the shoulder toward the chest.
Reshaping a shoulder means dismantling the jacket. It costs more than most jackets are worth. If the shoulder is wrong, put it back. Everything else on this list can be fixed.
2. The chest — the two-finger test
Do it up. You should be able to slide a flat hand between the lapel and your chest comfortably, with room for a fist behind it but not much more.
If the button pulls into an X-shaped crease, it is too tight. If you can get a whole forearm in, it is too big.
Do this standing normally, arms down. Everyone looks fine holding their arms out.
3. The collar — flat against the neck
The collar should rest against your shirt collar with no gap and no bunching.
A gap means the jacket is hanging off your neck, usually because the shoulder is wrong or the balance is off. Bunching at the back of the neck means it is riding up. Both are tailorable, but neither is cheap.
4. The length — cover the seat
Traditionally, a blazer covers your seat. A shorter cut is a deliberate style choice rather than a mistake, but the traditional length is the safe one.
The old test still works: with your arms hanging naturally, you should be able to cup the hem in your fingers.
5. The sleeve — show some cuff
The sleeve should end at your wrist bone, leaving about a centimetre of shirt cuff visible.
This is the cheapest and most transformative alteration available, and it is the one most people never make. A jacket with sleeves an inch too long looks borrowed no matter what else is right. Ask whether the sleeve has working buttonholes first — if it does, the alteration is more involved and costs more.
6. The waist — some shape
There should be visible suppression at the waist, so the jacket follows you rather than hanging straight. Not tight; shaped.
This is the easiest alteration there is. A tailor can take in the side seams cheaply, and it is worth doing on almost any off-the-peg jacket.
Buttoning
Two-button jacket: fasten the top button only, and undo it when you sit. Three-button: middle always, top optional, bottom never. Single-button: fasten it standing.
The bottom button stays undone. Jackets are cut on the assumption that it will be.
What to buy for
If you are choosing between two sizes, buy for the shoulder and the chest and alter the rest. Sleeves shorten, waists come in, length can sometimes be adjusted. Shoulders cannot.
That single rule will get you a better-fitting jacket than any amount of size-chart study. Why clothing sizes are inconsistent explains why the label is not worth relying on, and fit words translated covers the description.
For which colour to buy it in, colour analysis for men covers neutrals, and your colours has the palettes.