Then just close down your map best to close down the editor so you don't have anything loaded. Go to you project data and open the Map name you want to edit.
Search for the following
"height":
"width":
Right beside it you should see your height and your width like my 30x30 map says the following:
"height":30
"width":30
Change those 30's or whatever yours is into the size you want.
Open RPG Maker and boom!
Warning Large Sizes might cause the game to be unstable including the editor. I crashed with 5k x 5k.