You really should be a little bit more descriptive on what car your sr20det is going into.

Im assuming its an S13 240sx in which case you still have the 180sx front end on it (retractable headlights).

You should purchase the greddy or trust as known in japan front mount intercooler kit from yuri @ www.srswap.com . he can get it i believe for 1000 shipped to your house. It is the best fitting but you will need a custom turbo to first iC pipe pipe made custom out of mandrel bends.

as far as diameter i would do 2 ID from the turbo and 2.25 from the cooler this way the gas can expand and cool more.