figure this out in radiation to the load by compiling the radiators
based on 12 psi operating and 30psi relief
If less than 1,000 square feet of radiation - multiply the total by .03 for a steel tank
If between 1,000 and 2,000 use .025
if greater use .02
to convert this to use a diaphragm tank
tank your result and multiply times .55 if the building is two stories tall - or by .44 if three stories
The answer will give you the volume of the diaphragm expansion tank -- Manufacturers list volume in their literature.
This is likely over sized but it doesn't fail.