There was a utility for ToT that unlocked the map size limits in such a fashion, but there was a very practical limit on how big maps could be; Only so many cities could be built in total, so you easily ended up with huge empty continents.
It should be on Apolyton still.
Why would you just play the MGE edition if you have ToT?
Changing what a "variable" is in an exe is likely to not work well, since all of the code instructions are depending on absolute and relative instruction/data locations; changing a variable's size moves everything after it around.