Inter-coolers generally add more lag the larger you go, obviously the larger the intake and output, the less restriction it will cause, but the larger you go, the more it will cool or can cool, but remember, the larger, the more lag you will have
My intercooler on my SR is a 600 X 350 X 50 +/- and has 50mm input and outlet to match my boost pipes....
With an inter-cooler though, it really does all depend on how much power you are or do want to run..... But they are generally rated towards power figures so just ask the tuner shop what size intercooler would be sufficient for your requirements.
My choices were all made based on that I want very linear power for drifting, SR is also known for running hot so I went for a slightly larger inter-cooler to reduce as much air temp as possible too.
For normal road use you shouldnt need anything bigger than half my intercooler size on a CA, but if you want to do drags or drifting, one my coolers size would be about right, possibly even bigger depending on how serious you want to get.
But remember as well, it doesnt matter how large your intercooler is if it is sucking in hot air, it will obviously help, but a small intercooler with a cold air intake would possibly be twice as effective anyway than pulling in hot air from the engine bay directly.
Also once again, this is up for correction and further details I may have missed out if someone wants to add to it...