Hello, this is my first post. I’m new to unicycles and want to buy my first one. My inseam is 29" with my shoes on, 5’7" and 160 lbs. doing some reading according to my inseam I could use either a 20" or 24" tire depending oniv I select a 200mm or 300mm seat post. Now the question is how do I know which size seat post I should get? I don’t have a dealer closely so will buy it online but not sure about the size. I would like to eventually ride some of the flat sandy trails my kids ride their bicycles, but I think I need to start with a basic road uny correct? Any suggestions about the seat post size and consequently tire size? Any recommendations for a good beginners uny? Thanks
33 views and no reply? Ok so then how di you determine if you need a 200mm or 300 mm seat post?
You can cut (and you will) long seat post but it will be impossible to make it longer.
My wife’s uni (20" QA) arrived with two seat posts. Get longest one and cut for your needs as I did with my first seat post 
You can buy good uni and love it forever or get trainer and switch it later. 24 or 26" is good for learning.
At your height, you can ride any size unicycle you want. 20" or 24" are best for learning on, but from there don’t feel restricted.
Unicycle.com has a “Uni Guide” which includes a little table matching up seat inseam length with seat post size. Go to this page and then click on Uni Guide in the right column (the guide is a PDF).
The usual method of adjusting the seat height is not to buy a particular seat post size, but rather to adjust the seat such that your knee is just slightly bent when the pedal is on the furthest down stroke. That kind of adjustment would be for general road riding. So the adjustment depends on the crank length, the frame length where the seat post goes in, the seat post length, and the seat thickness or height. Like the other poster says, buy a longer seat post and have it cut to size.
I’m about the same height and weight as you, and you could ride a 29" unicycle or even a 36" Uni, but the larger size will definitely be harder to learn than a 20" or 24". However I think you would get more use out of a larger size Uni for road riding, since a 20/24" is going to be pretty slow and not very much fun for any sort of distance riding. You could compromise and get a 26" uni which still should have good speed but won’t be that hard to learn on. I learned on a 24" Schwinn (with a 300mm seat post) and it was OK, but it now sits unused since I ride my 29" and 24" Muni mainly.
When I cut a seat post, I usually use a pipe cutter.