From my two experiences in ballet with crash pads, (Swan Lake and The Firebird), my crash pads have been incredibly large. My Swan Lake fall was much higher than yours - roughly 13', and the pad used was at least 4' thick, roughly 8' long and 5' wide - a pad designed for a straight backwards fall (a foam material with canvas covering with a comforter on top to reduce the noise of the impact). When my company went on tour with The Firebird, we used an inflatable crash pad (very noisy) for a sideways fall that was only 4' off the ground. The pad, when fully inflated, came to at about 3' high. I can't recall the rest of the dimensions of that pad, since we had to cram it where we could, based on the theater we were performing in.
Assuming you mean the stunt person will teach the actor how to fall - that is a great asset. Unfortunately my ballet company does not employ stunt people or weapons experts. The dancers are just expected to do it, which is why I try to talk my dancers through any falls to help prevent injury. So assuming you have someone with knowledge of stunts/falling, they should be your first resource for what kind of crash pad is needed for your particular fall. If they are unable to give you recommendations on size/thickness/material - then I would strongly reconsider your choice in the stunt person.