I am trying to find out what the largest rim/wheel is that will fit on an 05' Subaru Impreza WRX STI and can't seem to find the information anywhere....anyone happen to know?

The largest we have test fit is a 19" with a 235/35-19 tire size. You can pull up all of the wheels that fit here:

http://www.tirerack.com/a.jsp?a=DY6&url=/wheels/index.jsp