There is no size limit on sprites, but each spritesheet needs to have a certain number of "components" to work correctly - and the $ and ! in the filename change how the spritesheet is processed by the game.
A single sprite needs 12 pictures of the same size (but that can be any size) - three horizontal for the animation, four vertical for the four directions. A full spritesheet needs 8 sprites in the usual 2x4 rows - resulting in a sheet of 12x8 single pictures of the same size.
If the filename of a spritesheet begins with $, then Ace will know that this is a single sprite and calculate the size of the sprite by dividing the horizontal picture size by 3 and the vertical by 4. Without the $ in the name the picture will be considered a full spritesheet and the basic size will be calculated by 1/12 of the horizontal and 1/8 of the vertical picture size...
You can look at the examples of the High-Fantasy-Packs in the store to see double-sized actors and much larger monster sprites.
If you follow the link in my signature, you'll find some tips explaining more about sprite naming.
The game's help-file also contains info on the formats used for sprites and tiles.
Just remember, if you want to use double-sized sprites you also have to look for fitting tiles as the size of tables from RTP will no longer fit the actors etc. And (depending on how large you will make the sprites) you might need a script to prevent wrong overlays with the tile above the original actor tile...