I know there's a way. In W3, some advanced editor was buggeable to get 480x480 (you couldn't edit camera limits to 512 because the window for that told you to downsize to 256). I remember making 480x480 maps. I think nowadays all AW, DAoW and LT use 480x480 (or maybe 512x512, no idea how advanced it was).
The "trick" is in altering the game to be able to have more freedom in 256x256, and get stuff like 128x384, and ultimately, 1x511 and 0x512. Once you get that, you alter some other thing, and get able to alter that to a bigger degree, making the sum not neccesarily a maximum of 512, but 1024. That's the way I see it.
Anyway, Blizzard sucks for making 256x256 the limit. With the way textures work now, that may screw the editor. I wonder if you can add more than 8 textures to a map... not even W3 managed to break the tileset limits.
If the editor breaks, but the game not, you could try creating the terrain with an image file (was doable in W3 but was hard, maybe it's in SC2, since you can convert SC1 and W3 maps anyway) and then spawning everything else with triggers; but that's pretty mad xDDD.
I made a map technically 2560x2560 by making all models 10% their original size. Be careful and reduce their movement speed and weapon range as well. Textures look a bit odd as well. Also, it crashes the game often. Try to only do 50% or no lower than 25% scale.
for one thing your math is wrong the map would be 281.6 x 281.6 or something close to this number. and if you are talking about 90% i think it would = around 486x486
256x256 = 65535
512x512 = 262144
65535 x 4 = 262144 ( see 512x512 is technically 4x bigger then 256x256. )
and you are also wrong with the scale.. I have scaled units down to 0.01(Smallest it lets you i believe ) scale and i am currently making a concept map.. which is a complete step past 0.1 which would be 100% smaller i believe so i am at around 200%.
I have not done the math but i think my scale is somewhat around 1024x1024
262144(512 total area ) x 4 = 1048576
This is how you determine area.
The amazing thing is with such a small scale the map actually runs better i average 125-150 FPS in a third person camera mode on ultra and my computer cant even normally run sc2 on ultra in a normal map :)