Coolant overflow capacity?

Real world experience-

My 73 318 Satellite did not come with an overflow/recovery tank despite coming with AC.
(It's also got a fixed fan, which is odd to me)

If I run the AC, it will get hot enough to purge some coolant, and after a few drives, the coolant level will be too low.

I added a 1 quart heavy plastic travel cup but found it would over flow when hot, and eventually leave the coolant level too low.

I found increasing to a 48 OZ container solved the issue.

Be sure the hose from the overflow goes almost all the way to the bottom of your recovery tank (effectively creating a "closed system" if kept below the water line in the tank), and that the radiator cap is a "two way" vent.