What is the difference between a 28" and a 29" unicycle.
They refer to the same size (ETRTO). Usually, narrow tires are referred to as 28", while wider tires are referred to as 29". But that’s just a label ; they fit the same rims.
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Maxence gave you the short (and correct) answer.
For the long answer and a thorough review of the confusing world of bicycle tire sizing, I recommend: Tire Sizing Systems
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The bike industry is causing the same confusion with the 27.5" size, which is actually exactly 1 inch larger than 26 inches (559mm or 584mm), so it should actually be called 27".
Funny thing: This size has existed for ages. My first unicycle with a large wheel 40 years ago also had this rim. It was called 650B and was classified under 26 inches. The bike industry then suddenly turned 26 inches into 27.5 inches with exactly the same rim.
There are also 650A (590mm) and 650C (571mm) as 26 inch standards. The 559mm of today’s 26" mountain bikes came later.
@Maxence is right about the wheels, but speaking about unicycles frame size matters too. Some frames cannot accommodate a plus tire.
This Flansberrium is a strictly 28" unicycle for example: Flansberrium unicycles - #363 by municycling
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Problem is there is already a 27" tire designation at 630mm ISO. So a 27" rim is larger than 28" and 29" rims at 622mm ISO, and 27.5" at 602mm. 27" used to be a common road bike size, not so much anymore.
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tire designations are a mess 
Never heard about 602mm ISO. Could it be 584mm one?
Sorry, you’re correct.
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1 inch