Somewhere out there I read the specific gravity of tap water at 80 C, say,
is 0.975 - c.f. 1. 0 at 4C. So hopefully 1L at 4C becomes 1.026L at 80C.

I'll guestimate 100L would be typical for a 'normal' system with rads +
indirect HW cylinder

So 100L over this range, 100L expands by 2.6L, 300L by 7.8L.

So 4 gallons looks adequate for a normal system, 12 gallons looks about
right for mine. 25 would be overkill, though I could set the ballcock
halfway down.

Hopefully I answered my own question? though I wouldn't know where to get a
12 gallon tank.

Egremont.