It's hard to say what scale this is. UCS? Special Edition? Minifig? HO? Who knows? Official measurements seem to be nonexistent, and unofficial ones range from 8 m to 20. Analysis of the film seems to indicate it's about 6.5 m.

I wanted to see what I could come up with building a less technicy Hailfire around the wheels from the official set. I think this is pretty accurate, but I'll confess right now that it's wobbly as an inflatable clown. Put it on a smooth surface and the wheels try to fall in on each other. Anyway, I really liked this custom when I started, but I've sort of gone off it, so it's a little less finished than what I usually show.

I put it under "modified sets" because I used LEGO's solution for both the wheels and the missiles. Plus it saves me having to figure out the scale!