There are six different tileset formats, and that format is more than just tilesize.
The five A# tilesheets are a combination of autotiles of different formats and regular tiles - and each POSITION on that tilesheet can only replaced with a different tile of the same type, not with other types of tiles.
The sixth format is the format for B to E tiles, each containing 16x16 tiles of 48x48 pixel. One of the Tilesheets needs an "empty" tile top left, that will be the very first tile on the B sheet.
Check the helpfile (F1) formore details on the different formats.