They are about half megabyte, file sizes vary so maybe with comparing them all you can find maximum.
update:
button thread was removed.
So the limit-button shall be this one then: http://i.imgur.com/mBo3VoJ.png
Quick reference:
Kernel Panic map 16x12
![Image](http://springfiles.com/sites/default/files/imagecache/file-default-image/images/spring/spring-maps/Direct_Memory_Access_0.5c_beta.sd7_minimap.jpg)
60 KB
8x8 map with very basic texture
![Image](http://springfiles.com/sites/default/files/imagecache/file-default-image/images/spring/spring-maps/layoutki63.jpg)
100 KB
Several speedmetal variants, about 16x6, 12x4 etc
![Image](http://springfiles.com/sites/default/files/imagecache/file-default-image/images/spring/spring-maps/1250612830.jpg)
290-500 KB (often including unneeded files)
Conflict Terra map 16 x 16
![Image](http://springfiles.com/sites/default/files/imagecache/file-default-image/images/spring/spring-maps/eureka_map.png)
536 KB (including features)
rules:
-smaller than button
-the best map wins
-until next monday or until there is winner