The most common haircuts on women with medium length hair today are bobs with layered cuts and lengthier front tresses. The main reason is that they provide the richest styling options to round face shape and are currently trendy.
The ideal length is over your shoulder or even longer. This particular length will elongate your round face. A plus for it are texturized edges. You can cover your forehead with asymmetric bangs or leave it open. But remember that you can certainly apply your own personal touch.
Women and even girls with round face look really attractive with a flat-ironed bob. One side of the face is covered by side parting as well as long side swept bangs giving your face a considerably thinner look. Also, if you opt for a straightened shag medium hairstyle for your round face with plenty of layers, this one will draw quite a few vertical lines to your face, and will finally add the desired proportional length to your hair.
But please be careful if yo have medium length curly hair. Simply because it often tends to be excessively voluminous and in the end it will broaden your face unfavorably. For curly hair it is recommended not to have it too fluffy if your hair happens to be teased at the roots. Having more volume on your crown will certainly correct those proportions.