Tires

As regards bicycles: when we speak of "size of the wheel" the approach used is not completely correct.
The size in inches 20" 24" 28" et.  is an approximate measure of the final diameter of the wheel.
For this reason, "chubby" tires, being wider and taller, increase the final diameter of the entire wheel leading to commercial definitions which are not always very clear.
I.e. the 28x2.35 or wider, is commercially called 29" but the rim is a  28".
On the contrary, the tire for Trial is called 20x2.5 because the final diameter is always around 20". However, in order to be strong enough, the 20x2.5 tire needs a smaller rim, which in slang is called 19".
The tires are measured also in "mm". This defines both the diameter of the rim and of the tire. The dimensions in "mm" are:

Below an overview on the wheel sizes