Well, I have the A530s. The platform is 3 3/4 inches long by about 3 inches wide. It has kind of an oblong shape to it rather than being a true rectangle. I've ridden to work using my running shoes several times and it's not a problem. I've never once found myself wishing for a larger platform.

Having platform on one side and spd on the other is a mixed blessing. As I've gotten more adept at using them I grown to like them more but having SPD on both sides would mean easier entry if you're using clipless shoes most of the time anyway.

Something like the A530 might be a good solution in your situation but FWIW I keep a pair of running shoes at work.