Don't make fanatically huge maps. Size DOES matter in AssaultCube, as huge maps will lag players down - not to mention, it will take forever for players to find/frag each other, in what is supposed to be a fast paced game.
WQD (world quads)
The WQD count can be found by typing /hidestats 0. Some figures will appear at the right of
your screen, one of them will be WQD.
WQD is a good way to measure how good your map will/won't lag. Your map should ideally average a maximum of 5000 WQD (less is better). It's okay for your map to exceed this if it's only a little bit more in a short area.
When creating a map, make sure not to make areas far too small. Six or more cubes wide is a good dimension for doorways, to allow people to pass through them easily.
Make sure players aren't "fighting the map" too much. They should be fighting each other after all. Be sure to make sure that people aren't constantly bumping their heads, especially against the skybox, as it can be annoying, and unrealistic.
Try to keep wall sizes about 3-4 cubes thick, to allow for detail and realism.
At the end of the texture list is a black/white cross sort-of texture. DON'T use it, it's not a texture, in-fact, it's lack of a texture. It looks ugly and shouldn't be used in any map, because it's actually there to show if textures and skins did not load properly.
If you'd like to create a map with friends, you can use co-operative map editing to let you build and modify maps with others online easily. This mode allows you to edit maps online with other players who could help speed up repetitive or boring tasks, bring in fresh ideas or spot errors you may have missed.
To start co-operative map editing, when in a server, type: /mode 1 and then type: /map and then your mapnames name and press ENTER. You will have to of made a "newmap" previous to entering the server to be able to co-op edit it.
You will then need to send the map. You can do this by typing: /sendmap and then your mapnames name and press ENTER. Note: Custom materials can't be sent, they will have to be downloaded another way if you are using them. If you are using a custom "cfg" file, it will automatically be sent with the map, if there is a cfg file present.
Note: Some useful editing commands can't be used in co-op editing, such as: copy, paste, slope, arch, undo, etc.