We can have a stab at it purely from an energy perspective. A defrost cycle for my 10kW pump requires around 1.2 kWh in heat that is used to thaw the ice (depending on outdoor conditions of course, but that’s the order of magnitude). Around 0.2 kWh of that come from electricity, the rest is extracted from the hot water in the system. 1 kWh is released when cooling 29 l of water from 30°C (my flow temp at 0°C) to 0°C. If you have twice the volume, you’ll only cool from 30°C to 15°C and so on.
From that energy you can thaw around 13 kg of ice which seems reasonable. Since it is not only thawed but actually ice heated to 0°C, then thawed, then water heated up it will actually be less mass you can heat up that way but the thawing itself is the by far largest fraction of it (the energy required to thaw the ice, i.e. transition from solid to liquid is the same energy that is required to heat water from 0 to 80°C!).
Taking all this into account I would say that anywhere upwards of 60 l would be a reasonable volume to allow safe defrosting for my specific setup. and I reckon it would be in similar ranges for other installations. You could do a survey on heatpumpmonitor to see how other defrost cycles behave in terms of total energy use and work you way backwards from there.
