Usually feet out (as the idea is to remove the leg drive part of the bench), but each grip should be used just like other presses (where close, medium, and wide would each be considered a different excersize with it's own PR).
You can bend your knees or just put them out in front of you, it shouldn't matter too much either way, I've done both. I don't think the floor press is exclusively used to remove leg drive, you can bench on a regular bench without leg drive.
all depending on the length of your arms of thickness of your torso will affect the range of the lift...additionally the breaking of the eccentric/concentric when the elbows hit the floor makes it a nice variant on the bench press. Not that it matters too much, just interesting to think about---it differs from a regular bench or board press in that the weight STOPS when your elbows hit the floor. It also differs from pin press in that you are still supporting the weight with your body and you have an eccentric.