When you buy the plywood, get three inches cut off each side, and that will give you an outside broder for the platform. 3'6" works best for me and for most platforms, as the rubber plates will often hit the border between the plywood and rubber with the full four feet wide. My first two platforms were four feet wide, though, and did fine; I did get enough damage to the plywood that after six months I changed it, so I'd go with that fromt he beginning.
A 4x6 sheet of rubber is standard. You can either get another sheet of rubber and cut the extra pieces from it, or get a half sheet of plywood and cut one foot by 2 foot rectangles from it and use them as a broder on the platform. I did this when I first built my platforms because I didn't have any rubber to cut, and the half sheet of plywood was only about $10 or less. I'd have them cut that in the store, too, since it'll be easy to assemble when you get home. and they can do it in less than a minute on their saw. Often they say they'll charge you for more than one cut per piece of wood, but it's never happened to me.