People          Size Gallons

2 or fewer    23-36
2-4           36-46
3-5           46-56
5 or more     60-80
If you have a large whirlpool bathtub you should add 25 gallon tank. 

The standard bathtub, which measures 60 inches long by 30 inches wide, holds approximately 35 to 50 gallons of water. Larger units, such as whirlpool tubs, soaking tubs or freestanding tubs, hold approximately 60 to 80 gallons of water. Spa or corner tubs typically hold 70 to 120 gallons of water.

a 10-minute shower consumes approximately 25 to 50 gallons of water or more, depending on the type of showerhead used.

Low flow shower heads will reduce the amount of hot water needed.
An older style shower head (pre 1992) can demand from 6 to 8 gallons per minute (GPM) of flow rate. A newer shower head (after 1992) usually uses 2.2 GPM.

You can increase the capacity of a tank by add an on-demand electric heater at the tank. This can increase the capacity up to 45% .
See Eemax AutoBooster

Links:
Buying Guide for Different Types of Water Heaters at The Home Depot
Residential Gas - Water Heaters -  The Home Depot