[chromo]

Well, speed is relevant to size and pressure to some extents.

Using say a 2000rpm drive, a 4" wheel has about half the surface speed as an 8" wheel and maybe third as a 12" wheel. 1 rpm is one full turn, so in feet per turn a larger wheel is moving faster than a smaller wheel on your part.

4" wheel X PI = 12.57" per rpm or about 1' per rpm. 2000' a minute at 2,000 rpm.
8" wheel X PI = 25.13" per rpm or about 2' per rpm
12" wheel X PI = 37.70" per rpm or about 3' per rpm

your drive motor must have enough torque to maintain speeds with your size wheel at the pressure you apply to the part. Press harder and you bog down the motor more.
So for a small light weight dremel tool with a low torque motor you need a small wheel and lighter pressure in order to keep up the RPMs, and if you bog down the motor with large wheels or hard pressure you'll burn it up.

That being said, 1/4" shafts for your rotor zip is not the problem, you need to figure out your RPM feet and Buffer wheel sizes for your rpm motor and torque. Any machine shop can turn a 5/8" bolt down to a 1/4" for you to fit your tool. Slap the buffer wheel on the bolt, tighten the nut, lock the 1/4" end into the rotozip.
In theory you can get basically any size shaft you need made then for about $5 maybe less at a decent machine shop. Be aware though, using a large wheel on a 1/4" shaft may bend the shaft easily or a little out of balance make it jump around wildly like a mexican jumping bean on steroids!
I turn stuff like this myself on my lathe all the time to adapt most any type part to any motor shaft I want.