I've frequently seen the MPC speeder referred to as 1/35 or 1/32 scale, which I think is based on the size of the pilot figures. However, the kit figures are very (very) oddly proportioned, and appear too small compared to photos of the studio miniatures or live-action setpieces. If you look at the cockpit footage from the movie, there's very little headroom, the tops of the pilots' helmets look to be within an inch of hitting the inside of the canopy. I've also seen a figure of 1/22.5 scale quoted, which sounds more reasonable to me.
I've been (slowly) working on an MPC speeder, and am converting a couple of Tamiya 1/24 rally car drivers into pilots. They fit OK in the cockpit, except there isn't quite enough headroom. I've taken out the floor of the cockpit and trimmed down the thickness of the seats, and I'll glue the seats directly to the inner surface of the model's underbelly. I'll also need to trim down the figures' posteriors a bit, but they should fit when I'm done.
I also tried fitting 1/20 scale F1 drivers - they looked good as far as leg and shoulder room, but were just too tall to allow the canopy to close.