My older 22x22 icon set of many more items and objects (Mega Mushroom still breaks the limit, I've since abandoned these in favour of the 25x25 ones in this topic, a few pixels can make all the difference). It's also the reason all the sub-types are not my first priority this time around. I will get to them eventually, though.
@ Francisco Cifuentes:
Working in 25x25 tiles seems like a good idea and would eliminate my worries about needing more space for larger items. I'll make a mock-up of a menu screen once I have some more of these done.
Some things from my dA gallery so this post has some content.