Standard is 20" but there are some 24" around. Your description is what I refer to as “unicycle with drop plates” (got a better name?) and I think would probably be best for riding a skatepark (although a freewheel unicycle is more versatile overall). The easiest way to build one would be to get a peg-style (vs. pedal) drift trike fork that already has a disc brake, cut a notch in it, and put a unicycle seatpost and seatpost bolt on. That’s what I’m planning to do to make a 3.8x geared freewheel unicycle. Another route might be trying to affix a regular unicycle frame onto a BMX wheel somehow. I’m considering making one but probably not until after Unicon.