tire sizing. The rim is 20.6 mm inside width, 25.5 mm outside width. ( I weigh about 170 pounds, and these are the tire pressures I use.)
GP4000S measures 26.6mm wide. These work great: I use air pressure appropriate for a 25-26mm tire. The rim is "tubeless ready" so the tire bead locks in securely (with a startling pop after installing a new tire!) I use 80-85 psi front, 95-100 psi rear.
GP 4-season measures 29mm wide. Very nice on rough roads at 65 psi front, 80 rear.
If you can measure your current tire width: You can use a hex L wrench to test the side clearance. A 4mm wrench is 4mm across the flats, for instance. If a wrench fits in the gap, you have double that size to work with. But you don't want to be too close to the frame!
The cheap Harbor Freight digital calipers
are very handy for all kinds of measurements.