Women: 2.5-3 sizes down from street shoe size
Men: ½ -1 sizes down from street shoe size
Pointe shoes should be quite snug around the foot and are best fit by an experienced professional.
When you have a good fit, the sides of the shoe will lay flat against the foot when standing on flat and there won't be any twisting through the box.
You should be able to feel the end of the box with your toes but not have them crunched.
If it feels comfy, it's too big!