Be realistic about the tank size. Your average shower uses 17 gallons of water. An average flush uses 1.6 gallons. You can cut that down by retrofitting one of those dual flush systems. If you are flushing 10 times between showers, you get away with a 20 gallon tank. A 20 gallon RV cube tank is 21" x 17" x 14" and might fit in the cupboard under your bathroom sink or ceiling mounted above your toilet tank.