I think that this is a good start, but I have a few suggestions:
1. The tile editor should show which tile is selected for placement.
2. It would be nice to highlight which tile the mouse is in, when moving around the actual map.
3. The ability to have differently sized tiles is cool, but its interface is awkward. The dimensions for map and tiles are different in the Edit dialogs, but nothing suggests that. Also, you have to click the tiles before seeing the effect of resizing the tile.
4. When placing the first resized tile, it seems to reset the map back to grass, with the new tile size. Unfortunately, it doesn't redraw the old map, so it looks like things are larger than they really are.
5. Text dialogs to resize things are tedious, they should be part of the interface to drag new sizes, or both dimensions should be in one pop-up.
I know this is a lot of feedback, a lot of which might seem negative, but I also believe there is room to make a cool application. I have 3 final suggestions that would help address my concerns listed above:
1. Make sure to redraw things when an event changes how they might be presented (tile resizing, etc.)
2. The ui could be more helpful, such as tile highlights, or text descriptions in the dialog boxes
3. Resizing the tiles should allow for tiles of multiple sizes, in the tile editor and in the actual map.
Regarding #3 I have some ideas, in another list
1. For the tile editor, instead of selecting a single tile from the list, a user could choose a box size and move it anywhere around the tile images to create a custom tile.
2. Then when they moved the mouse to the map, the tile square would fit within part of a fixed size map tile. When placed, that large tile would be split into 4 smaller squares, holding the existing images + new tile (the actual split depends on how you allow users to place tiles and customize them).