Generally speaking, it is prudent to allow 20 gallons for each member of the household.  Remember, in a single tank system, the tank acts both as a solar storage reservoir and a back up heat source. It is for this reason that a larger tank would be recommended. The advantages of a larger tank are two fold: Extra volume of hot water could potentially meet your needs on one or more subsequent cloudy days.  They also tend to mitigate overheating issues in closed loop systems. 

When considering solar collector sizing, we recommend 2 gallons of storage for each square foot of evacuated tube collector area.  For flat plate collectors, use 1.5 gallons of storage per square foot of collector area. These are guidelines and may be adjusted by your installer to suit your circumstances. 

For space heating and commercial applications, a careful system design must be accomplished as general sizing guidelines are not appropriate in these cases.