Hmm this is a tough one, because I have a round face but the classic rules of thumb don't really apply. I've even traced my face shape, measured it, had consultations, so I know its round, but volume and or height at the crown, and or short hair looks horrible on me. I think its because I'm short as well, so big-ish/voluminous hair only makes me stand out as petite. I found the perfect cut/style for my round face shape and small frame (slim 5'2'') is long hair (about nipple length or armpit length) with LONG layers, meaning the shortest layer falling at your chin or lower) cut and feathered with a razor (scissors will be too blunt), and keep the hair parted to the side.
Stupid me thought I'd try something different so I tried a new style (a little longer than the collar bone) with layers starting at my ear, and side bangs. Big. Mistake. Too many layers starting too high, so now it sort of looks like a mullet. The layers were too blunt, so they're really easy to make out, and the bangs (even though theyre not thick, theyre wispy and are pushed WAY off to the side) just don't work. I really wish I kept my regular, tried and true style
Hope you find what works for you!