The amount of water doesn't matter.
Only the surface area of the system matters, and the medium and temperature on the other side of the surface (putting your tank under ground is much better then having it in open air, for instance).
The amount of water matters only when it comes to how fast the system will react. If you have four gallons of water in the system, it will take a long time before you reach the maximum temperature (probably an hour or more), but you will reach the same temperature as a system with a quarter of water with the same surfaces, eventually.
I use two large Senfu radiators myself. The system contains about half a gallon of water. It's slightly too little surface to cool my system fanless (I've directed airflow through there, so it doesn't matter), around 4800 square inches.
Maybe get a large used car radiator from a junkyard if you want to experiment? Although they might require som work to adapt to a ½" water system.