Clothing, no matter what item, should fit. I can't express this point enough. When it comes to leather jackets, or any jacket for that matter, the general rule is the shoulder seam should hit at the end of your shoulder. Leather should be snug but not too tight. It should be fitted through the shoulders with narrow arms. Also keep in mind the length you're going for. Bomber jackets are very popular right now and they should sit parallel with the waist line. If you're going for mid length, then it should stop right below the hip. Never and I repeat never invest in an leather blazer, a leather jacket with extreme detail and/or stitching, or one that looks like you borrowed it from the 70's. 
