On my project, A Little Rage (link in my signature below), I originally planned the cab width to be 18 3/4".
I cant remember if I deviated from that plan, and since my grandaughter, who received it, lives 75 miles from me, I cant measure it now to be sure.
I may have increased it to 20" but not real sure.
Anyway, it worked out pretty good for my grand kids, which are still small.
I did have to use smaller buttons and a small track ball, you can check out the project thread if interested. The CP was pretty close to the edge on P2 side.
Looking forward to see what you come up with.