Technically you can go down to 1-13/16", but most use no less than 2" between centers. I have a double crossover that is 2" on centers between the parallel tracks, so that's one of my gages.
On curved track I let it range between 2-1/8 to 2-1/2, depending on the radii. The tighter the radius, the more clearance.
I don't know the width of your roadbed material, but I would think that would self gage, edge to edge. Put two pieces side by side and measure center to center. if it's 2" or more, you're safe.