Expansion tank sizing?

Hi guys, I am an engineering technologist that is new to the field. We are doing a retrofit on some apartment buildings and we need to switch over from an older style expansion tank (without diapraghm) to the new style. We were wondering if there is a quick easy method to use the old one's size to select a new one.

I posted this last night, but I think they lost it in the process of the upgrades they did on the site...check out this http://www.watts.com/pro/divisions/watersafety_flowcontrol/support/support_ETsizing.asp?catId=64
Basically, you need to know how much water is in the system and then how hot you will make it.

Oops, my bad, I forgot to mention that the main problem is we are trying to size it without knowing the sytem volume, thus the reason for using the old one's size.

Putting in an oversized one wont' affect system performance, but adds to the costs and space requirements.