I have just picked up ballet again, at the age of 20 and bought these slippers at a local dance store where I paid $20 for them. I wish I could have waited a little longer, I would have bought them online. I would suggest that if you have never worn ballet slippers before and you are not sure what your size is, to go to your local dance store and try some shoes out. This way you can figure out your size. I usually wear a 7.5 in streetshoes but need a 9.5 in these slippers!
Besides all this, these ballet slippers are an excellent deal. Great quality and excellent price. 

I bought these for my 12 year old daughter who is trying ballet for the first time. Because she wears a 9.5 in women's sizes, and has a wide foot, I ordered a wide, 9.5 slipper. These slippers fit as your normal shoe size fits. I should of got the medium width, because their wide slippers are very wide, and now it's a little baggy on the outsides of her feet. Although it does have the elastic ties, it still doesn't help tighten it to fit snug. So if you have a wide foot, just order medium width, that should be wide enough. Other than that, this slipper is durable, and looks great. My daughter loved wearing them, and is enjoying her ballet classes.

According to that chart, style 205 (this shoe) is one of the ones that runs comparable to street size. Best bet is still to measure your child's foot anyway. Might I STRONGLY recommend going to a shoe store so they can do it with their magical slidey foot scale.