The image attached shows the relation between lean distance and radius of the wheel. It shows that the more the radius of the wheel, the more you will have to lean forward to overcome an incline.

x - lean angle
r - radius

Why is the torque equated?
the torque that the wheel produces creates a reaction force on the frame of the unicycle. Taking free body diagram of the frame, this torque is balanced by the moment due to the rider forward

Why is this important?
This is another constraint for wheel diameter.

Credits - John Chew( from Facebook unicycle group)