This is something that’s fairly subjective these days. In recent years, jackets have become shorter and closer fitting at the waist, while trousers have become much slimmer in the leg and shorter, with some men preferring to show some ankle.
However, there are certain areas to always look for. For example, your jacket should sit perfectly between the points of each shoulder. If it’s too tight it will be restrictive and stress lines will appear across the back of the jacket. Too big and it will hang off the shoulders and you’ll look like you’ve just started school.
The chest area is just as important and most men are in between sizes when shopping. So a 43″ or 45″ can become a 44″ or 46″ (standard chest sizes go up in increments of 2″).
With the chest you want it to fill the jacket without the lapels bulging and sitting away from your pecs, which would indicate the jacket is too small. Nor do you want the appearance of too much fabric, which would obviously mean it is too big. That being said, as I mentioned some men prefer more room while others prefer less and at the end of the day you have to feel comfortable.
An area that certainly isn’t subjective is the balance of a jacket. Of the thousands of men, I’ve measured over the years, very few have shoulders angles that are evenly matched, usually the dominant hand is more sloped.
An easy way to check this is by simply looking in the mirror with your jacket on and seeing where the top button and top buttonhole meet. If they are level then your jacket is balanced, if not then it is unbalanced.
In some cases the difference is too small to matter but often when you button up the jacket a line will appear in a diagonal from the top button. Occasionally the lapel on the side of the shoulder that slopes will buckle and sit away from the shoulder.
If you have square shoulders, even if they’re even, a ridge can appear across the back of the jacket. This can also appear if your jacket is not balanced from front to back based on how you stand. For example, if you stand up very straight, this has to be taken into account as less fabric will be needed on the back of the jacket.
If your tailor isn’t using a tool (see below) to measure the angle of each shoulder and the way you stand, then the jacket won’t be cut accordingly.