Getting different sized wheels to run at the same speed

I planning on using 75mm wheels with 44 tooth pulls direct on the drive motors but want to use 100mm on the back

They will be connected via timing belts and im guessing I would need 55 tooth pulleys on them yet ive got a feeling they don't exist in T5x10mm

Reasons for the different sizes is that its needed to make the machine invertible, plans for a vertical crusher

Make all wheels 100mm

What's the question?

Thinking I might just go with 4 100mm wheels, make everything easier

it's easy. Wheel 75mm is 25% smaller than the wheel of 100mm. Therefore, to keep everything equal, the pulleys need to differ the same amount.

In short, why ask, as you had the answer.