Below is just a little brief guide can not cover everything as so many issues to cover but hopefully give a few people a few pointers when choosing.
People often base their purchase of an intercooler on the biggest they can get in the space needed. But this is wrong, some of the main things to consider are
What power you will be running
Flow of pipework inc diameter and length
Cooling capacity and heat transfer
To use a Bar and Plate or a Tube and Fin core
The aim is to go from hotside (turbo) to the coldside (throttlebody) in the shortest distance this will prevent lag, ideally with the least amount of bends.
If and where possible its best to keep the diameter of the pipework no smaller than the turbo outlet or the throttlebody. Too big and again can add to lag. to small and can restrict the airflow.
The core needs to flow and fill the core and cool the charge. a common misconception is to put the biggest core possible in. But you must consider the output of the turbo if its only a small turbo a small intercooler will perform best. Bigger turbos will require bigger intercooler.
Other problems to consider is if core is too thin or too tall. A tall core hidden behind the crash bar will not get the best cooling.
Bar and Plate cores are more commonly used as they take longer to heat up, which is fine for fast road use, unfortunately take longer to cool. So for extensive use in competition tube and fin cores are more used.
The complex nature of the fine fins means the temperature transfer is faster. Also another thing with competition is weight reduction as Tube and Fin cores tend to be half the weight of a Bar and Plate.
Maintenance is also worth considering, if a piping kit is made of mainly silicone over time boost leaks can happen. Also silicone swells under boost, a kit made from more hardpipe allows the boost to flow better. Aluminium is recommended to make the piping from as is light. I have seen people use steel but this is very heavy.
Also remember to bead your piping this will stop the pipework and silicone blowing apart. Just simply by putting a lip on the end of the pipe, We can do this for you.