Yeah, the bigger it is, the slower it heats, and cools. But, in my opinion, I think its better to use a smaller tank. They are easier to fit inside your case, if you ever need to move it around. They give more accurate temps sooner than larger tanks. Also, smaller tanks can be easily made from tupperware (so can larger ones, but really large tanks are usually fish tanks or something, so they're $$) The radiator is what really deals with the heat. You should get a good heater core, and shroud, with an intake and outtake fan, or run your lines through a freezer
Almost forgot, a bigger radiator isn't always better, for the same reason that two or more radiators aren't better than one. Your water system, especially with a 200gph pump has very low pressure, and water takes the path of least resistance. What this means in a large radiator is that not all of the space will be utilized, so your cooling won't be much better than if you used a smaller one.