Hello,
Technically, you can use the whole 32-bit color palette for your images, and sprites size have no limitation. However, you must be aware that by default, the game uses a 16*16 pixels collision box for the hero and most entities, so if you want hi-res sprites, you'll need to use Lua scripting to change the entities' size as well as their origin point (which also defines their map coordinates). This includes their sprites too, to keep things aligned.
However, doing so is likely to cause collision bugs, so be careful to stay in multiples of 8 pixels for the collision boxes (sprites don't matter).
Tiles can also be of any size as long as they respect the 8-pixels rule (the editor forces you in this direction anyway).