external wastegate sizes depend on a lot of different factors...what kind of turbo, where is it being located, is it a log manifold or tubular, etc etc etc...my guess is either a 35 or 38mm wga...srt-4's with 500+whp are using 35 or 38mm...only reason to go to a 40 is if you have about 600+whp...
The way you control it is with a boost controller. Also, you want the wastegate to open up all the way at the VERY LAST second before redline...or before your car shifts under WOT condt's... Reason being is if it opens too early then your wasting all that boost and not getting the most out of the car/turbo setup. If it opens up all the way at about 2-300rpm's before redline then your using all that power and putting it all to the ground, and not wasting much if any of the boost.
BTW, if you externally dump it to the atmosphere (vs. dumping it into the downpipe) its 50 state ILLEGAL, and any inspection place can fail you wit ha visoal inspection...just an fyi...so look for a manifold that has the external wastegate dumped back INTO either the manifold or downpipe. Also, you'll want the wastegate to gather exhaust gases from ALL THE CYLINDER PORTS!!! not just 1 or 2...ALL OF THEM!!!...
Probably more info than you or anyone else needed to know but hey...better to live and learn the experiment and pay up the @$$ for repairs...