How to choose the right size?
The toes shouldn't be able to move around within the shoes: they should be held in place without being too tight. To be confident in your choice, when trying them on, stand up and test a few movements like flex/point, and stand in demi-pointe.
All demi-pointe shoes have a tightening cord to make sure they can be perfectly adjusted to your feet. Once adjusted, the excess cord should be cut off. The remaining length should not hang longer than the shoes, nor be made into a loop.