Yes, indeed that question  came up recently in the work I'm
doing . . . so far as I know, it's not the limit of the message  box
or userform  which is usually the problem . . . the problem is the
space you might want to put it into, such as a cell or on a page or
on another userform. With my most recent effort, I used a 200
character limit for "comments", but of course that was constrained
by the cell it was being transferred into, limited by the printing
space. I needed to be able to put those comments for 5 call-data
lines onto one page and it worked with that limit.
If you don't have printing limits per page or not those limits,
then you will likely succeed with larger parameters.