Short answer is : yes, the client supports larger maps, but you have to be careful when you make them!
2013-07-08 :
Maximum map size : 1500 x 1500
2010-07-30 :
Maximum map size : 1350 x 1350
These large maps take about 300 MB of memory, but I doubt people will go that high.
Apart from the client version, maps also have a limit on the number of "lightmap" references, which is 65536 (ushort). Each tile is associated with a lightmap. Let's assume you have a map of size 512 by 512, that would mean...
Tiles Up = Width/2 * Height/2 = 256 * 256 = 65536
Maximum Tiles = 3 * Width/2 * Height/2 = 196608 (up, left and right)
Lightmaps to fill = Maximum Tiles = 196608
That means that even a map of 512 by 512 can "break". If you don't use walls, then you only have 65536 max tiles and there are no issues in that case. You can reuse an existing lightmap (which gives you a much bigger margin to play with), but BrowEdit doesn't seem to support this when you recalculate the lightmaps (this is the common "Sanity error: lightmaps out of bounds" error people get).
Just for a quick comparison, the map schg_dun01 has 63854 lightmaps and it is 400 by 400 in size. The map has many lighting effects though and the complex background is what increases the amount so much.
Also, maps larger than 1350 require a minimal size of 80 MB (this is without lightmaps), which is unfortunately too much for cps.dll (the limit is around ~75 MB). I doubt this will be needed anyhow.
For further reference, these are the maps I've used to do my tests ingame (they do not have lightmaps for the reasons mentionned above) :
http://www.mediafire.com/download/737b0u8dp9b6v3f/testmaps.grf
You could have maps of 2500 by 20, there are no problems with these for the client. I don't think BrowEdit is able to calculate quad trees which aren't in a square shape (I may be wrong on BrowEdit's capabilities though! I haven't tested with non-squared maps).