Somewhere along the line I did measure it, though I've forgotten, but it is indeed somewhere between 1/18 and 1/20.
Which makes it a great starting point for similarly shaped but larger "championship" cars of the period in 1/25 or 1/24.
I'm pretty sure somebody on this board has built one.
Just as an aside, I have a vacuum-formed kit of the 1941 Indy winner co-driven by Floyd Davis and Mauri Rose. The kit came to me with no packaging, so I don't know its maker, and the one-sheet line drawing of the real car that came with it doesn't specify scale, though it's clearly 1/24 or 1/25.
The point is that held side by side, it looks very much like the old Monogram Midget body was the basis for the buck the vacuum shell halves were pulled from.
I also have a built-up chrome-body model that appears to be a midget, but I've been unable to find any info on it to date (though I haven't tried very hard).
EDIT: Found it.