How do you know what size intercooler to run in a certain application?
Surely it's not a case of bigger is better, as wouldn't the larger it is be the more restriction it is? Also would it depend on the application, e.g. wouldn't the air coming out of a turbo be hotter than that coming out of a supercharger, as the turbo is closer to the hot exhaust manifold? Does it depend on the amount of boost being run (or is that only a consideration when checking if the itnercooler can handle that much, e.g. saw one on eBay.com.au saying it can handle up to 75psi)?

A few rules of thumb.
Run as big as you can that will not affect your engine temps.

Most of the cooling happens across the firs 20 to 30 percent of the core. After that the temp barely changes.

Always try and go for a thicker core.

That pressure reading means that the core can handle 75psi before the welds break. People think that they will never see 75psi but what happens if you have no blow off valve, run 30 psi and snap the throttle shut after a run (as with most turbo drag cars). The pressure build up is immense.

Restriction is an intersting point as lots of things affect restriction. Most notably being the tank design and pipe diamter.

also it's the compressing of the air that produces the heat isn't it? not the fact it's close to the exhaust.. yes that fact it's close to the exhaust does affect it.. but i think it's the compression of the air that produces the majority of the heat..