Ummmm... is that it? No crafting? Just a single sentence? Also im pretty sure there was a test mod like this.
You can craft a shrink ray and there will be certain small mobs that are half of a block
and you can still jump 1 block and there will also be a grow gun that makes you 4 blocks high and you can jump 3 blocks and there will be certain items with how tall you are
normal: everything normal
special armor and items
possibly an item that when small you can throw an item at the wall and go up blocks like a ladder but ladders dont work when small or big