Well, in my opinion LV makes HUGE bags, so I always opt for the smallest size (PM). They don't make all three sizes in all bags, so sometimes you don't get this much choice. The largest size (GM) is ALWAYS really big and the MM is the middle size. It depends on how tall/large you are and how much stuff you need to carry.
I never buy a LV in GM. Cause it is really huge! The MM size is already big enough for me. IMO, PM = small, MM = medium/large GM = GIGANTIC! Hahaha. I'm not a huge fan of large bag or oversize one. All my bags are just medium or small. That's why I get my speedy in 25 & Alma in PM.
In French I believe PM is Petite(small), MM is Mediam (Medium) and GM is Grande (Large). Im sure I ruined the French but that's basically what the letters stand for. Not all bags come in each size. Some bags come in all three, but you just have to research the bag you're looking for.
yes i agree that the gm is totally huge. depending on the style of the bag you'll either opt for a pm or mm. or depending on your lifestyle, maybe a gm! i try to downsize on what i carry around with me so pm or mm is good enough!