Ditto on what Andreas said. As big as you can fit as long as it's bar-and-plate (no one sells tube-in-fin anymore) and the end tanks are relatively curved. $100 giant eBay intercoolers all day long. Easiest purchase you'll ever make.
As for Johnny wangwang's comments about lag. I'm glad he brought it up, because it gives me another opportunity to educate, and debunk this horrible myth. Johnny, can you quickly explain why you would experience slower boost onset ("turbo lag" isn't really correct here) with a larger intercooler?