There's no limit, and it doesn't have to be square. However, you should be aware what the consequences are of making the map too big or too small.
WarLight will size your map to fit the player's screen. Because of this, the things that appear on the map (army numbers, attack arrows, deployment numbers, etc.) will size along with the map. This means if your map is really big, when WarLight sizes it down, the arrows/army numbers will get sized down too.
One time I saw a map in testing that was over 13,000x13,000 (really, really big). It fundamentally worked, but the army numbers, attack arrows and such were so small that they were almost invisible. This obviously isn't desirable, so I resized the map down.
Similarly, if you made a map that was too small (say 100x100), it would get enlarged and the attack arrows would be insanely big.
You may already know this, but in case someone else is reading: Svg is a vector format, which means the size doesn't indicate the amount of detail that it can contain. It's not like working with pixel-based images, where the dimensions define how much detail you can include (pixel images can only fit one color per pixel.)
In vector images, you could in theory have a 1x1 image that contains the entire Earth map - and it would look perfect when zoomed in. So the actual size doesn't really matter - I would recommend just setting the width/height around 1000 and sizing your content to fit within.
Does this help? If you have any more questions let me know!