How big should an eleven year old girls breast be?

This is not something that you have to have people on the internet solve for you. It's very easy:

There is no perfect bra size for an 11 year old. It varies with everyone. You might have twin daughters and one wears an AA cup bra and the other an A cup bra. You have to figure it out for yourself. There is one easy and accurate method. With a measuring tape, put it right beneath your breasts and write down the number and add 1, that's your band size, put the measuring tape around your chest over the nipple and write down the number. Subtract with your band-size and what ever number comes out is your cup size.

Example: If your band size was a 34 and the chest size was a 33 you subtracted and it came out 1 inch your breast size is 34A

Mathematical example:33 + 1 = 34 - 33+ 1 inch = Size A cup, Band Size 34

Notes:

This is in inches.

You might need to add 2 instead of 1 for your bandsize.