If you want the system to remain sealed, I believe that the headspace in the purge can should be roughly 6% (or more) of total water volume in the system.

        The water will expand about 3% as it goes from room temperature to operating temperature. The air above the water will go from 14.7 psia to (at most) 25 psia. This means that you can compress the air in the headspace to just shy of 1/2 its original size before you push some out of the cap. Thus, you get less than 1/2 the headspace to use for water expansion. This says you must have at least 6% of the system volume for headspace in the purge tank for the system to remain sealed.

        For a 4 gallon sealed system,  you need a quart of headspace. Otherwise, the system will not remain sealed, but will spew on warm-up and draw back in on cool-down.

        You can build a system with zero headspace, but it will draw in and out of the overflow tank. This is the way nearly all modern cars are set up. Older cars are set up with the proper headspace and no overflow tank. On a 4 gallon system with zero headspace, you can expect about a pint of overflow once the engine comes to temperature.

        The sealed system with headspace is heavier, since you need to include a large pressurized container, (as opposed to a small un-pressurized container) but it is more reliable since you are not depending on the proper operation of the radiator cap and over-flow system.