If you created a map with the default dimensions of 96x96x32 and suddenly find yourself wanting more room, fear not! You can adjust the size of an existing map in any direction:

/ADJUSTMAP <direction> <amount>

Adjusts the size of the map in the direction specified (N, S, E, W, U, D) by the amount specified. Amount can be positive or negative. Extends or retracts the boundaries of the map to allow for expansion or to remove unused space. The amount must be a multiple of 32. (The Chunk size) Always make a backup of your map before using this command as it cannot be undone.

(Excerpt from Counter Spell Wiki[counterspell.gamepedia.com])