The length of your blazer will dictate how “balanced” your upper body is to your lower body. The blazer should end around mid to lower crotch. Mid-crotch if you’re under 5ft 9inches, lower than that if you’re taller than 5′ 9″.
A tailor can shorten your jacket up to an inch without messing up its proportions, but they can never really let much out because there’s no fabric there. Overall, you generally want this part to already be perfect when you’re buying a blazer, even off the rack.
Blazer Sleeve Length
While wearing the blazer – when you bend your wrist, so your palms are facing the ground, the sleeves should be about ¼” above the top of your hand. A lot of people recommend that it hits the top of your hand, but that’s bullshit – then none of your shirt sleeve will be showing. This length allows for a little bit (¼”) of your shirt sleeve to peek through. If your sleeves are longer, a tailor can easily fix that. If they’re shorter by more than an inch, ditch the jacket, the sleeves can’t be let out enough to fit properly.
You want the shoulder seams of the blazer to end where your shoulders end – where they start curving down to your arm, basically. You should see no divots or wrinkles in the shoulders anywhere. The shoulders should lay perfectly flat, with no divots or rumpling or pulling on the shoulders. If you have more rounded shoulders, the seam should still end in the same place, you would just need a little more padding in the shoulders to make them appear less rounded. If the shoulders are too big or small, a tailor will have a very hard time fixing this, and it would be very expensive – if it was even possible. So ensure these fit properly when buying a blazer from anywhere, as well.
With the top button fastened (never the bottom button), the blazer should lightly hug your midsection, but not feel tight or constricting. It shouldn’t be pulling at the button, creating an ugly “X”. The X means it’s too tight. If it’s roomy around your stomach/waist area, you can (and should!) have a tailor take in the sides of the jacket so it fits properly. This is a very easy and common fix for a tailor to do. Remember: For tailoring purposes, it’s better to have a jacket that’s slightly too big in the body than too small.
The collar should rest against your shirt collar, which in turn should rest against the back of your neck. All of these should touch lightly, without significant gaps in between. If there’s a gap, it’s too loose. If there’s bunching just under the back of the jacket collar, it’s too tight or the stance of the jacket is off.