With a condensing gas water heater w/~75KBTU/hr burner you should be able to get there with a 50 gallon. For non-condensing burner (any rating), 65 works, but if it has less than 50KBTU/hr burner you might have to wait for it to recover after filling the tub before somebody in as-yet built second bath could take a shower.
If you wanted margin, the 100KBTU/hr burner version of the condensing 50 gallon Vertex has a higher first-hour gallons rating than most 75 gallon tanks. You may have subsidies available to you for going with a condensing water heater too. A 100K condensing burner can run ~3gpm (about 1.5-2 low-flow showers) of 110F water forever at typical Seattle area incoming water temps.