The standard seatpost on a unicycle is 300mm long. We do sell them in lengths from 200 to 700mm in our spares section.
On a unicycle the seatpost will give you adjustment of about 100mm, this is due to a lower limiting factor of the seatpost being directly above the tyre. To solve this, you can either buy a shorter one or cut the seatpost down to size. This is a standard thing to do and will not void any warranty.
The simplest and cleanest way of doing this is with a pipe cutter. Measure how much you need to cut off (keeping this to a minimum) and mark. Gently tighten the blade of the seat post cutter to your mark, clamping it in place before rotating the seatpost. (Holding the handle of the seat post cutter turn the seatpost towards you otherwise it can spiral and damage your seatpost.) Tighten the blade gently every turn.
Alternatively, you can use a hacksaw and then file off any burrs so that goes into the frame easily.