You can't change it for the normal number box, but you could use the "nbx" numbers (called "Number2" in menu) instead, which offer the same settings as sliders, toggles, ...
yeah, the size of the box can be changed, but when playing different instruments and maybe acting as well, it could be a help if the number itself appeared in that size, in same cases maybe over the whole screen.. [i think of showing bar numbers for different orchestra groups like in the opera of lachenmann].
joh.