Dimensioning of solar thermal system

We are planning a solar thermal system with 5 sqm on the roof and a buffer tank of 300L. The water will be used for domestic hot water preparation. My questions: a) Is it worthwhile to go for a larger tank considering the collector area and also use it for heating? The storage volume is not fixed yet. b) What about circulation: keyword Legionella. What type of buffer tank is recommended here? The collector area is fixed (I will not go into details). Under these conditions, what is most effective and should be considered in terms of safety aspects (e.g. Legionella formation)?

We also have the 300 l. There should be no legionella, the water is clearly sixty plus hot. Heating support is nonsense. In summer you don’t heat, in winter there is hardly any solar yield. Leave it, waste of money.

Is the buffer storage with corrugated pipe or tank-in-tank? Which one is better? What about the risk that the liquid in the heating circuit heats up/overheats if no water is drawn? I have heard that many systems get damaged because of this?!

I don't know any of that, they installed it, I just use it.