In the SC2 Galaxy Editor, you can change your Map Bounds and Camera bounds to corolate with that:

Map -> Map Bounds
A minimap pops up with camera and playable area bounds.
When "Modify Camera Bounds" and "Modify Map Bounds" is selected, the two will expand proportionately on designated side.

You can click on the little arrows to extend the map, which extends it X amount.
X can not be changed.
One "arrow click" extends the map by I think around 6 cells? Left, right, up, or down, respectively.

Is it possible to change "X"? - I only want the bounds to extend 1-2 cells, but you can't type in the amount of increase manually.

No, you can't even do this when creating a new map. All sizes are 8 apart.

ew, alright thanks Temp!

Just gratuitously extend the map size and then limit the actual playable area with unbuildable/no pathing/cliffs/etc.