What race for WvW/PvP?
Racials are pretty much always worse than profession skills. Play what you think looks the coolest
Anything but Norm.
The fact that my arm length was bigger than my skill actual range kittened me off so much that I remade my thief from scratch.
Charrs/norns might feel clunky to play due to their size. I have norn and human thieves, and although I like norns, I prefer how human thieves move.
Norn Snow Leopard racial can lead to some pretty awesome fun in small roaming groups, particuarly if you have a group of four or even five norns all pop it at once.
Aside from that, Asura would be your best bet due to their size.
The only racial skill that I could consider using with my thief would be the sylvari rooting skill. That immobilize for 7sec on a 45sec cooldown.
Asura if you want everything you do to hardly be visible.
Norn for snow leopard, really useful mobility skill.
Human to simply look awesome.
Sylvari looks really good as thieves as well and also have a useful racial or 2.
Also if you enjoy fooling people, create a huge norn or charr and people wont recognize you as a thief at first.