I would like to argue for small not being an advantage particularly because I intend to play norn -- well maybe, their run/jump animation bothers me a little-- and I hate asura. However in practice it does seem to give an advantage. It seemed easier playing as asura or even sylvair than norn. People can't see too well what you're doing. You can hide easier behind terrain. It's not *as* obvious what wepaon you have (& hence what spec abilities you have etc). You do seem to get attacked less. This is gonna matter more the more zergy it is.
This being said i choose based on aesthetics
-the weapons scale with character size so asura cant get huge weapons
-asura look like rats
-sylvari are adorable and I love them however their legs look a little weird so my light armor classes can be sylvari since i want my warrior to show leg ><
-human female have elongated legs and 16yo faces, unless this has been improved lately
-char: too animal like for me
-if i intended to roll a male character i wouldn't roll norn, or only for comedic value since they have ridiculously wide frames and can wear leapord skin
-additionally: (heavy armor is the right). I need this armor. Or at least the helm lol