I did some research not too long ago, so here's my input:
Each manufacturer has it's own criteria,
Spidi split its line of clothing to sport, naked, custom & enduro. The sport line is tight fitting naked is loose. Leathers that'll fit both categories are somewhere in the middle.
Alpinestars tell you in a 7 grade level how the fit is between relaxed & fitted, the sleeve profile between straight & 90 degrees curve, the ventilation low to high and water resistance 0% - 100%.
Each manufacturer has it's own parameters.
Avoid buying without trying on first, or at least make sure you can send the leathers back if you don't like the fit or to change to a different size.
Best companies in my opinion are: Spidi, Dainese, Axo, Alpinestars, Arlen Ness, Berik and Spyke. (I think Berik is sold in the USA in limited form under the brand name MOTOGP, the logo looks identical).
A 2-piece suit is a good idea, you can mix sizes for top & bottom pieces.