Hmm, your problem seems to be that you can't set the map size in pixels but rather have to set it in tiles. If you want your tiles to fit the screen exactly, you'll have to use a tile size and a tile count that makes this so. For example:
Map size: 80x45
Tile size: 16x16
Gets you pixel size: 1280x720
The default tile size is 32x32 because that's a commonly used size, but you can use any size you want.