MarkMy 33.5 has an Edson arm that the gps mounts to. I went with a 5"plotter although I could have went with a 6" unit. The boat came with an old Garmin 400 series plotter and the new garmin 5" was pretty close to the same footprint and I just felt that anything bigger would be cumbersome . The 5" has been discontinued by but you can still find them in stock at a few places online.
Imp, I mounted my Garmin 942 where the compass is and I like it! It swivels so that anyone in the cockpit can view it. I can post a picture if you have an interest.Trying to figure out how and where to mount GPS at helm without it being in way and getting wacked around.