From garmin.com,

GPS V = 5.0"W x 2.3"H x 1.6"D (12.7 x 5.9 x 4.1 cm)

SP III = 3.2"H x 6.8"W x 2.6"D (8.0 x 17.3 x 6.5 cm)

SP III gives you a color screen, verbal alerts (not just beeps) and a more comprehensive list of dstinations (food, lodging, attractions, stores, etc.)

Maybe it's possible to have a 7" display pop out of the cubby like the nav display in a RR or Bentley.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD