You can probably get a general handle on it from the various dimensions knowing single or doubles but a possibly easier solution below.
You need the EV capacity (which you have) , the EV precharge and filling pressures (at the boiler, cold) then with all rads etc on and system up to normal temperature note the boiler pressure, and the flow and return temperatures, easy then to calculate the system volume reasonably accurately.
It would/will be quite interesting to see what you come up with but you certainly won't go far wrong with the 50 litre EV as you can't oversize it.
You need the EV capacity (which you have) , the EV precharge and filling pressures (at the boiler, cold) then with all rads etc on and system up to normal temperature note the boiler pressure, and the flow and return temperatures, easy then to calculate the system volume reasonably accurately.
It would/will be quite interesting to see what you come up with but you certainly won't go far wrong with the 50 litre EV as you can't oversize it.