Whatever makes the wheels move exactly one revolution.
That sounds like a cheeky answer, but it’s true - without knowing how many microsteps your motor controller is configured for, and what (if any) gearing is present in the rotary, you won’t get a number as an answer.
In the rotary setup dialog, choose an arbitrary value to start with, like 10000. Set the roller circumference to 100mm, then exit the rotary setup. Draw a tall, thin rectangle 100mm tall, and run it. There will be a pause as it draws each of the two horizontal lines of the rectangle, and the rotary will turn for the two vertical lines. It should rotate a full 360 degrees, pause, rotate back again, pause again.
If the two pauses don’t hit the same place on the rollers, you need to adjust the steps value. If it didn’t make it 360 degrees around, increase the steps number. If it went more than 360 degrees, reduce it.
Once you have the correct steps count, then set the roller diameter to the proper number and you’re done.