It would be smaller but servos any smaller than what i'm using are very expensive and not to mention rare, using the smallest standard size lets me use more for cheaper. So a bit of a trade off where that is concerned, however i might make a smaller one in the future that doesn't have as large range of movement. Also I am aiming to make this only require a 3d printer and the electronics to make, I could more than likely design something that uses additional hardware (screws and so on) and it would almost certainly be smaller. I want Hexy to only require the electronics and access to a 3d printer.
I do have some ideas for future projects and lets just say they are incredibly small, I have a few micro stepper motors that are smaller than an eraser on a pencil.
The only limit is the one you set.