If Liz has a slight advantage crouching, IMHO it's not by much. If anything, Major's crouch is even thinner than Liz's, depending on where the legs are.
(You can mess with the cg_thirdperson settings in a devmap game -- far away, 180 degrees. You can use certain walls / textures as a "you must be this tall to ride" sign.)
OTOH, note also that the player models are not (yet?) to scale -- Beret and Major are taller than Sergei and Grism, and Sorceress and especially Angelyss are very tall in comparison. Tony is very
short (sorta counting the hat -- he'd probably have to wait and meet the others after the ride). As far as size, he seems to have the biggest advantage (smaller target to aim at, also with backwards jump). (I don't know if people use Arachna.
(Look at the player selection menu, or again use cg_thirdperson, 90 degrees or something.)
Aside from the size difference, crouching isn't something I've noticed very much (maybe Grism / Penguin). (What about animations?)
Other possible advantages: Ayumi and Gargoyle may be easier to spot underwater (boots, wings), and some darker skins (Tony, red Sergei) might be better off on certain maps (but everyone uses brightskins now, don't they?).