WEU is a piece of junk and just makes the map file bigger every time you save with bloat.
Use JNGP, I think its no limits feature allows the exact same map size (480x480) without adding the bloat.
Sadly 480*480 just has a large map size (starts at 1 MB about). This is because of the size of the war3map.shd, war3map.wpm and war3map.w3e files due to the map demensions. A 480*480 map has 4 times as much terrain data as a 240*240 map. The size only gets worse as you paint more complex patterns and pathing on the terrain due to fall in compression effectiveness.
A 960*960 is not possible in editors. It will probably even crash the game (game not designed to deal with so much data). You can try making one by manually editing the war3map.shd, war3map.wpm and war3map.w3e files in the map archive (all have to match the size of the map you want otherwise it will flag as map data curropted). This would need a lot of binary data modification and possibly heavy scripting.
I advise sticking to 480x480. Infact I would recommend an even smaller map. Such large maps are very hard to fully utalize as the game engine is not designed for such complexity. Further more the time needed to reach an acceptable quality will probably exceeded your interest in map making resulting in eithor an incomplete map (areas unused) or a rushed map (areas not used effectivly).
To coclude.
1. Use JNGP, I believe it can also break the map size limit (and does atleast break other limits).
2. Consider starting out as a small map and expansing it as needed. It is better to have a little map with good utalization than a almost emptry map or poorly used terrain.
Most maps using the 480*480 end up sparcly populated (large areas of open space) or even not complety allocated (regions of map which have no purpose).
