Most, if not all, 1/8 buggies use a standard size servo. The size is really of no concern for that reason. You need to make sure you get a servo with metal gears and something with enough torque to turn the wheels. Something with at least 120oz and that's pushing it. Something in the 200oz range would be better. If you gotta go cheap, I have heard good things at the track about the Hitec HS-5625-MG. It's got 120oz and it's kinda quick.