I decided to buy 2 stall mats, and will be going with a 6'x8' platform with a 40ish" center piece, and the rest occupied by stall mats.
Seems a bit silly to buy an extra stall mat for what amounts to 6-8" of extra rubber (width-wise), but I figured I wanted to do it right the first time.
The one thing that sucks is the freakin' OSB I bought came with this blue spraypaint or something on the edges of it... and it's been getting all over my basement. I'm going to have to wrap the "base" part of my platform in plastic to keep it from staining my freakin' carpet. Such is life.
I'll be putting it together this week while my wife is out of town... I'll post my results.