I did the following: copy a blank map in the Maps folder, open it with .rar/.zip, extract both files, edit with Notepad++, and change all heigth and width  values with my desired number. After that, replace the files within the map file. I know the current max stable size has been proven at 186x108, not sure why. I have just published my Cisplatina II map for download which is 190x110, you can download it and edit to save any hassle, but im still testing the size, it seems to work, with a few hang ups here and there.

I will later today try to create a 220x130 map soze, but  not sure which is the height limit. I tried making a 220x150 size, but the map failed to load the top part of the map, so the height limit seems to be around 110~130, whereas I still have to find the width limit, which should be around 250 or less, but these sizes seem to be too heavy for a normal game, also it can easily extrapolate the 254 territory limit.

I would do further  testing. But have very limited time.

Nevertheless, the devs should definitely add at least another giant size to the generator with a fair warning that it may be too resource intensive for lower end pcs.