Too often people just buy a random size of intercooler (or turbo for that matter) without actually sitting down and doing the math to figure out what they actually need.
Not to be johnny raincloud or anything like that, but you need to read some books. "Maximum Boost," "Turbos, Superchargers, and Nitrous, "Turbos and high tech performance," and "How to Tune and Modify Engine Management Systems" are ones that I have on my shelf and have helped me learn a LOT about this kind of thing. Am I an expert? By no means. But I have a much clearer picture of what I actually want, and what I need to do to get it.
You need to get a target hp figure in mind, find out what turbo you'll be using and figure out it's efficiency at whatever pressure you want to be running, then find an intercooler with a pressure drop/efficiency ratio that you can live with to achieve your horsepower level.
If that is too confusing, just buy a random one that fits.
People sometimes get the wrong thing and it can turn into an interheater in the right (or wrong) temperatures, and then sometimes people also go WAAAY overkill and get the ZOMG!! 4" racing quadruple core JDM FAST unit when they really don't need it.
Usually, you can buy intercoolers based on their hp. rating, so you can also just know that, and take that route.