Try to think of it this way,

The more boost you make, the more exhaust you make, which makes more exhaust, which makes more boost, etc. etc. All the wastegate is is a valve that opens when you hit your desired boost level and allows the gases to flow around the turbine, instead of through it. It has to be able to flow enough gas to let the turbo slow down so if it doesnt, then you get boost creep where boost levels slowly grow if the car remains under boost. Its not really a matter of getting 10psi here, throwin away 5psi there, more or less having the turbo flow at the boost level you choose constantly. The wastegate controls how much boost your turbo makes, your turbo doesnt make a certain amount of boost and THEN expect the wastegate to dump it.

As far as what size to get, depending on your boost levels you should choose what size accordingly, I have seen 35mm's on high horsepower cars, but then again bigger is sometimes better. Get a little more precise with what your setup is going to be, and Im sure the right size wg will come to you.

Props to a fellow b16 soon to be whipping some ass, keep us updated on this project Im sure theres more b16 fans who would love seeing this kinda power coming from the motor. Happy boosting