How big is a size 28 AAA bra?

A 28A bra would normally fit a ribcage measurement of 28 inches or just under, and a bust measurement of around 29 inches.

The number is the "band size" or "back size," and refers to the size of your frame excluding your breasts. The correct band size will usually be your underbust/ribcage measurement rounded to the nearest even number, for example if your underbust measures 33 inches you will most likely need a 34.

The cup size is represented by a letter, and this is relative to the band size so if for example you currently fill a 30A but you need a 28" band, then a 28A will be too small. You will need to try a 28B - go up one cup size for every band size that you go down.