Why are foot-long pedals not widely used?
I'd wager the two biggest reasons you don't see foot-sized pedals are the increased rotational weight, and the difficulty you would have catching the pedal with your foot before it struck the ground or the front tire. I'm sure someone tried this once and promptly scrapped the idea after the foot-sized pedal struck something.

Pedals need to be stiff and durable, which is fine. But if pedals had to be stiff and durable and large, they would probably end up being heavy, so it makes sense that most pedals you see are small and compact. Shoes already do a great job of interfacing with and conforming to your feet, and it's easy enough to make the sole stiff with plastic, fiberglass, and carbon fiber. All of these factors combined explain why we have small pedals.