For me, I wear a size 38 in Chanel and a size 37.5 in other brands like Christian Louboutin and Roger Vivier. I also know another person that goes likes to go up a half size in Chanel.
As for the Chanel ballets, I actually find the toe box roomy, which is why I love them. I use ballets as a walking shoe, so I literally walk around all day in them and they feel great. The cut of the vamp (is that what it's called... the toe part) is nice because it covers enough vs. other ballet that have a very tiny toe part and barely cover anything which them makes your feet look like they are plump and oozing out of the shoe (I hope that made sense!).
On the cambon style, well... for me, it is cuter on display than on my feet. The toe part covers too much; it goes up nearly 1/3 of the way up my feet. And the leather is a thick leather, add on top the CC and it looks very bulky. I love shoe and have tried it on several times (as if my feet were going to look differently in it one day!). But the last time I tried it on, I was with my husband and asked his opinion and he said he made me look like I had really swollen fee due to the way the top is cut. So for me, it doesn't work. I expect it will look great on other folks.
I think ballets look good a tiny bit on the loose side, they look very carefree and chic.
I hope this was helpful.
