...
I think that for historical reasons, all the variables that hold those
measurements are 16-bit signed integers. The largest value that fits
in such a variable is 32767. When you translate that number of twips
to inches, it's 22.755 inches -- so I think that's where the 22-inch
limitation came from.

Ok, I think your guess could be right.
But... this way of programming was just bad IMHO. I understand i.e. when
someone programming uses a 32 bit variable for a memory size (just an
example), with a limit of 4GB, when memories at that time are smaller than
20MB. Obviously the limit will be reach a day in the future, but it is
comprensible at that days.
But when MS programmed Word, there ALREADY existed A0 paper format (over
55cm limit!), so if they choosen 16 bit variables they intentionally left
out that large paper formats. (sorry for my bad english...)

Microsoft could change the declarations of all those variables to
32-bit signed integers, which are supported by all versions of Windows
after Win95, and accommodate page sizes of more than 5800 meters!
However, they would also have to comb the code for all the things that
depend on the current definitions, which would be a massive job. I
suspect that they've considered it and failed to find an economic
justification for expending the time and money.

Yes, I think it's a very expensive task too, with no economic advantages
because there are already other programs to work with large paper formats...