As an undergraduate, I use the 6"pads. While I like the shape of the edge on the larger pads, I wonder how they would work on the sides of a Boxster. My experience is that even the 6" feels a little too large for the curves of the door area. I sometimes actually drop down to a spot polishing pad for the bottom of the doors.
Check my work here:
http://986forum.com/forums/showthread.php?t=3413