How Do I Design a Hot Water Storage Tank?

You need to design it in conjunction with the solar collectors.
After all you can't store more than the collectors produce!
Google solar hot water systems...there is tons of stuff out there.
you don't say where you are which doesn't help, if you are somewhere sunny you will need a smaller tank as it will be constantly replenished.

There is a rule of thumb that says something like 1square metre of solar collector for every 1.5 people or something like that.

it also depends on what sort of system you want to build ... and what is available.

This link shows what I did.

Are you going to build the tank or use what is commercially avialable?
I'd suggest more basic research...then ask a more specific question.

Del

I would buy a readily available 100g. hot water heater from HD, wrap an insulation blanket around it and be done. An advantage of this is that in a week of very cloudy weather you can connect the gas or elec heating device and have backup hot water.

In fact if you build your proposed hot water tank and if 4 people take 2 baths in one day, then what happens if it is cloudy for the next three days? Also are you serious about baths as opposed to showers. Filling a bathtub 8 times a day will be a hell of a lot of hot water and keeping it hot until finished with the bath. Not very practical for solar unless you have a field of solar panels.

Or have you summarized the situation and it is actually 2 people taking 1 bath for 4 days.

Try using software found at http://www.retscreen.net/ - it will give you a starting point to go from...

Oversize the solar collectors, make sure that you can mask off several on days when you are getting too much hot water, use a heat pump to improve collection on cool, sun free days. Use antifreeze to stop rust and prevent freezing in winter.

90°C. Isn't it a bit hot? My hot water is set to 60°C and that is plenty hot. You should use less energy to heat to a lower temperature.

The most efficient water heater tank that I could find was made by Reem. It is called Marathon, and has a plastic water tank. I sprung for the 85 gallon model. It is a little bit more money up front, but has a lifetime warranty. Set the power supply so that it will only be active after the setting of the sun. Use a low temperature setting on the thermostat. In that manor the solar panels will warm up the water on sunny days. but if the sun does not come out, and the day has ended, with no hot water stored, the electric heater will come on at a lower temperature to provide some hot water during the evening-night bath cycle. Good luck.