Your best shot would be cutting out the two bottom pieces, placing them up against each other on the floor, and positioning your foot on that.
If your foot goes beyond the outer edges in any way, then you betcha' it's small.
It should extend a bit out from your foot, as you will probably want to use a shoe in there instead of just wandering around bare feet or in your socks, and seeing as you'll need the space if you're going to fiberglass it.