On my 95 - they were 4" front, 5.25" rear and front door.

Personally - if you have 4 speakers, I would use a 4 channel amp, or at least 2 2channel amps. Otherwise you will be very limited on how you can adjust balance between the speakers.

__________________
1995 E300D - 225,000 miles - White Exterior, Grey Interior - original owner