I’m a new rider, but I believe I may have purchased a uni much too big for me. I am 5’5, and was recomended the 20’’ wheel. However, my legs appear to be fully extended at the lowest pedal, does anyone know which size would be more appropriate for me, and where I can go to trade in my bike?
Thanks
Don’t panic!
I am 5’7" - only 2 inches taller than you, and I regularly ride a 28 and a 36.
The height of the seat is of course easily adjustable.
However, you may find on some models that you can’t lower the seat far enough if you have fairly short legs. The bottom of the seatpost may stick out and hit the tyre.
The simple answer is to take the seat post completely out of the frame and cut a couple of inches off the bottom of the seatpost. This is easily done in a few minutes with a junior hacksaw. (Or, indeed, a senior one.)
Ideally, take any burrs off with a round file. Hey presto! Your seat now goes lower.
Re: size of wheel
Yeah you can surely lower the seat, but…do you mean trade in your unicycle?
Re: size of wheel
This may not be a bad thing! How extended are your legs? At the lowest pedal position, your leg should be fully extended and your foot at a similar angle to what it would be when standing on the ground (so you shouldn’t feel like you are on tippy toes). Many beginners have there set level set way too low.
If you are felling over extended then as previously suggested, a hacksaw can be used on the seat post to shorten it, allowing it to go down further. Bear in mind that your legs should be extended in the lowest position when you shorten your seat post so you do not take too much off it. It is easy to cut bits off but not easy to add them back on!
There is also a nifty device called a pipecutter which is even easier to use than a hack saw, but most people don’t have access to one unless they know a friendly machinist. (You can buy them for around $50 or less if you plan on shortening a lot of seat posts 
The height you can lower the seat down to is limited by the unicycle frame. Once your seat is sitting on the top of the frame you will not be able to put it down any further without modifying the actual frame. Given your height the only kind of unicycle that would cause you a problem is one with a long stem frame. Even a 36" wheel is easily rideable at your height. Note unicycles with long stem frames tend to be more expensive to buy so you will probably know if you own one. What model unicycle did you purchase?
actually i personally like to put my seat at least 2 inches lower than “recommended” so more weight goes on my butt:D but i ride long distance,so…