For one reason or another, I have usually boiled by water in order to purify it. I would obtain a large container suitable for boiling (I believe the largest I have is about 18 L - 20 quarts). I have many smaller ones as well which I often prefer for ease and safe handling. Your container should match your stove's capability - don't try to use a large container with a small heat source.

I sometimes boil a small container so that I can immerse it in a cool water bath in order to reduce the temperature. Having a big pot just gives you more options.