So I have been unicycling only about 2 months, but I am absolutely in love with it. I have been learning on a 20", and I am ready to look at buying something bigger. What I don’t get is the wheel/size vs. tire size thing. I apologize if someone else has already answered, but when I search for this, I just got a lot of discussion about the ideal size wheel for various types of unicycling. My specific question is, if i buy a 26" unicycle, does that mean it’s a 26" rim, just like a mountain bike rim and I could fit a regular mountain bike tire on it, or does that mean the outside of the tire makes a wheel that is 26" and it takes a different sized tire? And/or is a 26" unicycle with a big fat tire on it a much bigger end product that I am expecting (and I am going to fall an kill myself on it). I hope my question makes sense!
A 26" unicycle means it’s a 559 ISO rim, just like on a mountainbike. You’ll find that what you think you know about mountain bike rims is wrong, 26" on a mountainbike means 22" rim. Just like on a 26" mountainbike, the outside of your tire (depending on it’s width) will be roughly 26" inches.
See here for more info on Mountainbike sizes. Unicycling uses the same tires, rim diameters and terms, so yes, a 26" unicycle tire can be interchanged with a 26" mountainbike tire.
You are highly likely not to fall and kill yourself on it, but that would hold true for any wheelsize including 36" and larger and giraffe unicycles. Fall-yes, die - very unlikely, injured - can happen. But I don’t think riding a 26" will increase your your chance of getting injured by too much when compared to a 20".
and the x2.3 or x3.0 is about tire width right, not the thickness from the rim to the outside, but horizontally?
It’s a rough estimate for the width, although that does depend heavily on the rim you are using. But since the tire will approximately have a round cross section, a wider tire is also taller. When you compare a 26"x2.1 and a 26"x2.8 on the same rim, the 26"x2.8 will have a slightly larger rolling diameter.
(When I say rough estimate, that’s because especially when comparing across different manufacturers, a 2.8 and a 3.0 width can be the same, or even the 2.8 slightly wider.)
lol, I figured your rough estimate was based on how much air is in the tire. With less air the tire will get a bit wider than when fully inflated. 
Thanks very much
Thanks, this helps a lot!
The OD of the tire can vary quite a bit depending on the width of the tire.
My 26" offroad unicycle has a tire so big that the outer diameter actually winds up being about 27.5"
The larger tires roll over bumps so much easier that once you get rolling, it’s a lot easier to ride but you still land on your feet every time.
26" is really the best size because you have your pick of the litter for tires and usually for a good price