If you open the area file with a GFF editor, you'll see a width and height entry. You can then change those numbers to suit your needs. But that is not all. You then need to fix the tile list. If 16x16 is 256, then you need tiles for 0 to 255. So calculate your last tile name by (width*height)-1.
If you are subtracting tiles to make your number, this should be easy, as you can just delete the branches you don't need. If you are adding tiles, you need a GFF editor which allows you to clone branches.
It may be possible to cheat the toolset, but I have not yet tried this. What I mean is you might be able to just add the tile entry branch, and then set just the tile_id node to 0, omitting the rest. The toolset makes up for many parts of missing data by adding them when you change something. This might be one of those times.
I'm using GFF editor 1.3.4.7 and it has a copy/paste node/tree function in the Edit menu. You can also right click to delete node/tree. It should have everything you need. When you copy a node and paste it to the tile_list tree, it will put it on the bottom of the list, and give it a strange number. I copied 186 as a test and pasted it. The number was then 001, not 186 or 256 as expected, so be careful.
If I could, I'd make a suggestion to whoever takes the time to make odd shaped areas outside the 32 limit of the tileset, and that is to pack up an ARE file for each of your odd sizes and put it on the vault. That way a user can just go in the ARE file and change the tileset used. I'd suggest setting every tile to tile_id=0, assuming every tileset will start with either a ground terrain, or blocking terrain, open tile.