Drill a hole in the center of your wheel that is small enough to stick the screwdriver in that can tighten the servo screw - but is not so big that that the screw falls thought it.
Place the servo screw through the hole in the servo horn and then screw the servo horn to the wheel.
You should now be able to put your screwdriver from the outside of the wheel, thru the wheel, and then adjust the servo screw into the servo.
So you shouldn't need sizes - its the size of the screw on the servo and the size of the hole in the horn.