What size water heater needed for Jacuzzi?

I plan to install a 55-gal jacuzzi tub. What size water heater do I need, gas, tank?
4 people in the house, 1 bath, might add 3/4 bath in 5-10 years. I'm thinking 75-gal, but that might be over kill. Maybe 65 would cut it fine?

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.

You could also install a second 40 gallon tank in series with your other tank. I did that and we have lots of water. The nice thing about 2 tanks is that if one goes, and if you installed bypass valves, you will still have hot water and you have some time to replace the failed tank.

2 cheap forty's. Always best to design with backup, and can turn one off when the JAQ is out of service or you go away.