Your intercooler is going to flow a certain amount (CFM) of air. There are two important factors CFM notwithstanding that you have to consider. 1: efficiency and 2: pressure drop.

The former, efficiency is pretty basic.
Temp In-Temp Out
------------------------ X 100 = Thermal Efficiency
Temp In-Temp Ambient

Example:
Ambient temp = 86F
charge temp = 230F
outlet temp = 108F

230F - 86F = 144F inlet rise temp
230F - 108F = 122F drop
122F/144F = 84.7% EFF

Make sense? Obviously more efficient is better


The latter, pressure drop. When you increas the volume in the intake tract, your going to lose pressure. There's more vacant space for the turbo to pressurize and overcome, so you MAY or may not induce extra lag. An enourmously oversized intercooler WILL indeed lose you power over a proper intercooler by this effect, but a properly sized intercooler will usually net you similiar response to a stock piece, if not better.


Another consideration is the design style of the intercooler. Most factory pieces are tube/fin designs which is a cost compromise. See picture below.


The superior design, would be a bar and plate design (look at the intercooler and you understand where it gets that name) because it is more efficient, and has a lower pressure drop then a tube/fin I/C of the same size.


Best thing is to just do your homework. Bigger isn't better, there are just too many factors to consider. Remember, this is a supporting mod. It may make 10hp over your stock intercooler on a dyno, but on a HOT day when you're really beating on your car, a quality intercooler will keep you from heat soaking, and the benefit will increase two, threefold over a stock piece.

In other news, I had one of these show up today for the sky, 680cfm of love... 45% larger then stock and bar/plate