The bigger the better, even if the opening is smaller then the cooler. As long as you have stuff ducted and sealed right there is only one way for the air to get from the front of the vehical to under it, and that is threw the radiator and what every coolers you have up front. Just look at a factory Corvette setup, their is no main front opening, but a fish mouth deal just before the front end air dam. That tappers up to the size of the radiator, and as fare as I know this cools at around 200mph on the road course at BIR in Minnesota here.
I would just monitor the IAT going into the TB, make sure that is around or below 100 deg and you should be good. Also making sure you have a dense enough core with enough turbulators to transmit the heat from the air charge to the cooler you can make up for a lack or surface area up front for cooling. Just look at the stock Galant VR4 intercooler and the massive air flow that receives behind the factory bumper.
~John