What car you got mate, the bolt thread will be either M12 x 1.5mm Pitch Or M14 x 1.5mm Pitch, the length is normally the length of the thread, not the entire bolt
The reason for fouling on the back of the hub is probably due to the Chamfered (cone) style bolts sit further into the wheel than the Radius style bolts. Are the wheels definately the cone style hole, drop one of the bolts inside the wheel and press it firmly in, there should be minimal or no movement left to right on the bolt when its seated properly, If the cone style bolt wobbles about a bit then try the radius one in the same hole just to be sure
