Syntax: MAPD width(integer) height(integer)
This sets the dimensions of the map to (width) pixels wide and (height) pixels tall.
The practical limit of this command is somewhere around 650,000 by 650,000 pixels (attempting to make the map bigger than this causes out-of-memory errors and makes the game crash). However, as metaroom makers are still filling up an expanded 100,000 by 100,000 map, this is not likely to ever be a real problem.
mapd 100000 100000