I think the biggest single map we have sofar is in the Case Blue module. I haven’t actually counted the dimension in hexes!
Otherwise, I don’t think we’ve found the actual size limit yet. The upcoming new version of Vassal will be even more space efficient. Be aware however that the bigger the map area gets, the slower performance will be.
There are two different things relevant for map size: hex grid size, and
map image size.
If I remember correctly, we store hex coordinates as ints, so you could
have a map with 2^31-1 rows and 2^31-1 columns. You’ll have no trouble
Where you might have difficulty with is image size. The largest map to
date that anyone has made I think is the Case Blue map, which is which
is something like 11000x8000 pixels. There’s no limit on image size built
into VASSAL, but arrays have integer indices in Java, which limits image
rasters to 2GB each, so you could have at maximum an image which contains
535298048 pixels, which is about (about) 23170x23170 pixels.
I do not recommend making an image that size, however, as hardly anyone
has enough RAM to load such a thing. Instead, you should chop your map
into multiple smaller images, and form your overall map from several
Boards. (The limit on Boards is also 2^31-1, which you’ll never, ever