When does a 20" trial unicycle for somebody im thinking about buying one but i dont know if im to big im 5ft 11in and 140lbs am i to big?
No
Although some sites list uni wheels as being suitable for riders of certain size, that isn’t how it really goes.
Unless you are buying for a very small person, or child, 16 " is to small. From there, buy according to the type of riding you want to do. A 6 ’ person should buy a 19 trials for trials or trick riding. A 24-29 muni for off road riding, and anyone > 5’ tall can ride a 36 on the road with minor adjustments. So buy the uni that matches best the kind of riding you want to do.
Thanks