Oh, boy....
Uh, turbo "size" is not the really the main issue here. What you need to do is enquire about the correct centre section and then the appropriate housings for your application. The most important factor is going to be turbine A/R if your basic turbo sizing is in the right ball park. The turbine A/R determines most of the "spool up" characteristics of the turbo, though the compressor A/R, wheel trim, and size is also important.
Generallly speaking "larger" housings will cause the turbo to spool up later (ie: at higher revs [more correctly this is related to exhaust gas pressure and volume]). An example would be that a turbo with a smaller A/R number would spool up faster than a turbo with a larger A/R number - the trade off is that you will will lose top end with the smaller A/R but have a fast spooling characteristics. The larger A/R would give better top end at the cost of low revs or bottom end - in good turbo selection, these are balanced nicely for the application and intended driving style.
The compressor wheel size and trim does play a role, but as I mentioned before, it's of somewhat lesser importance than the turbine A/R. A larger wheel and a larger housing will ultimately flow more air and at higher pressures, but there are always trade offs...
There's lots of good reading on the web. It's a bit confusing at the start, but it makes sense once you've learned the terminology and can start to read a compressor map.
~John
