26 x 4 fat tyre

there's not an exact number; it varies depending on the design choices made by the bike builder or wheel builder

this page (random google find) shows some ranges for rim width vs tire size, if it's useful
 
30mm to 100mm. The narrower the rim, the higher pressure must be used at the low end to stabilize the tire laterally.

Ask yourself why you're using tires that fight all your attempts at forward progress.
 
if you mean the erd, (so you can get the right length spokes, etc) that is specific to each rim. if it's not provided by the rim manufacturer, you'd have to measure it; there are a number of ways to do that:
 
30mm to 100mm. The narrower the rim, the higher pressure must be used at the low end to stabilize the tire laterally.

Ask yourself why you're using tires that fight all your attempts at forward progress.
I ride in deep loose stuff from gravel washes to sugar sand... just doesn't work with 2" tires at 65psi (I'm typically closer to 10psi)... on soft terrain the rolling resistance is not any worse (and at times likely better) on the wide tires. And the good wide tires still handle quite well on hard surface at those low pressures...

Different missions require different equipment. I wouldn't expect a tour de france guy to run 4+" at 8-10psi nor to benefit from full suspension...oh well..
 
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