Solar Tank Size

One 30 tube panel should be sufficient for a house with 3 people (approx. 10 tubes per person, should provide 30% to 60% of the total yearly hot water. This will be approx.. 100% in the summer and 30% or less in the winter.
The 30 tube panel is suggested with cylinders up to 260 liters / 60 - 80 Gallons
If your cylinder is larger then it can be heated with more than one panel. It is possible to add an extra panel to an 80 gallon tank if you aim to benefit from higher temperatures in the wintertime but it must be designed for coping with any overheating that is likely to occur in the summer.

This is a very simple answer to a big, complicated question!  

For a longer and more detailed discussion detailing sizing your solar tank visit here - Pressurised and un pressurised solar tanks.