Tires are sort of a mess of standards. Sorry about that, we're stuck with 130 years of very slow change. The easiest way to figure out tire sizes is the ISO 5775 or ETRTO standard. It'll give you the bead diameter, which needs to match fairly exactly the place on the rim where the tire get held in place, and the width of the tire (which isn't really millimetre-precise). The numbers look like 35-622 moulded somewhere on the tire, and mean Width-Diameter, in millimetres. Right now the most common diameters are 622, 584, and 559. You regularly see 590 and 630 as well, and sometimes 571 and 635.

If you hear inch-sizes or things like 700C or 650B, those usually refer to the outer diameter of the tire when mounted on the rim. Vital for framebuilders, but not actually that helpful to match a tire to a rim, given that there are something like six different and incompatible tire sizes called 26".