solar powered Hydronic heating- storage tank capacity.

I am planning to fit such a system here in Bendigo, Australia and am wondering what size to have the 'heat storage' tank for overnight use.
I am aware thermodynamics experts would be able to work it out, but I was never an 'expert' just a drifter.
Has anybody done it before?

I'm curious to hear from others as well.  

These folks do free quotes, perhaps they can give a ballpark size estimate.  Or perhaps looking at their social media for finished projects can give a rough size idea:

https://www.solarhydronics.com.au/hydronic-heating-system-design-advice/

Based on some notes I have from a Living Web Farms DIY solar workshop video, I think the water itself is circulated at 90-120F (~32C - 49C).  The video is freely available online, but it covers more of how to build solar collectors and normal solar hot water heating, rather than sizing storage tanks specifically for hydronic heating.

P.s. Just wildly guessing here, but one idea would be to somehow track how much energy you currently use each night to heat the space in another form (electric/wood/gas), then convert to Joules, and then perhaps calculate the equivalent mass required for that same heat energy stored in a water tank using a temperature differential between room 22*C temp and storage tank of 49*C.

George, I used to fall asleep in the Thermodynamics classes at Engineering school!
But I have mates who did not and they may be able to help!
Somebody in Australia says he uses a 5000L tank.
I have plenty of flat plate solar water collectors so its possible.