6 foot 3 and still growing, what sort of unicycle would you recommend for just leisure riding on the street/sidewalk? Does weight count as well? I’m 170 pounds.
I would say either a 24in nimbus isis or a 29in nimbus isis
can you already ride?
I’d go with the 24in nimbus isis. more possibilities.
Yeah, I can ride already, but I would be able to go alot farther and for alot longer if my uni actually was my size… 
How far do you want to go is commuting your thing? Do you wan to also learn some tricks? Name some primary uses and how far you want to go when leisurely riding. A 36"er might also be something to look into for you.
im 6’1-2", 70kg (150lbs) and i use a 20" trials, but only because im into street and trials
if you only want to ride the thing and maybe do a few hops for curbs and such, a 24 or a 26 nimbus should be ideal.
The challenge in fitting taller riders can be in finding a long enough seatpost. You’ll be able to fit any standard unicycle as long as the seatpost is long enough.
If you’re tall you’ll want to look for frames that have a longer neck (the neck is the tube that the seatpost fits in). A frame that has a longer neck means that you can use a shorter seatpost. A frame with a long neck may be the only way you’ll be able to fit a 20" unicycle.
For example, there are long neck 20" freestyle frames. That will be the type of frame you’ll need to look for if you want a freestyle.
Unfortunately the long neck frames aren’t so common. Finding a long neck 24" or 29er won’t be easy. You just might need to get a custom frame for that.
You can extend seatposts by getting someone to braze on a sleeve and an extension.