Initially my platform was 4'x8' MDF with a 4'x6' stall mat on either side. Left a 2' outcropping of MDF on which I set my squat stands. After about a year I narrowed the 4' wide MDF to 40"+-. The narrower width promotes good rubber on rubber contact, but either width will be fine. I didn't (and don't) have any issues when the bumpers land off the rubber.
I didn't want to mess with cutting the stall mats so I've left them intact. It makes the platform more spacious.