Hi
For domestic solar heated water using flat plate collectors around 1 sqf per gallon of water or 50 L per sqm of collector area is commonly used, this will supply you with all your hot water in the summer months. It makes sense to provide for 2 days of storage to gain max benefit from the sun. So the tank size really depends on what your daily draw off is. I dont think bigger is necessary better - higher losses, more tank insulation is required, expense.
If you plan on a 1500 g tank then if it located inside the building envelope any heat lost goes into your house.
With a wood boiler, I would make the main HWC sized for 1 days draw off, then you can fire up the boiler for a couple of hours and have a days worth of hot water to use. Any solar input could also feed into this tank and then when it got to useable temp divert to a larger secondary tank as a pre-heater.
Cheers
Mike
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