Oval is considered the ideal face shape because it works with most lengths and haircuts. The oval is proportional and balanced, it is typically 1 1/2 times as long as it is wide, and the forehead is a tad wider than the jawline. If this describes you, you can wear super short crops, very long hair and everything in between. If you face is oval and petite avoid super full, big volume hair it will overwhelm you.
Square is wider than it is long, this shape is often associated with a prominent square jawline. Your best bet is to create a slimming effect via the illusion of more length and less width. Try cuts that are longer than chin length, so it draws the eye down. For short hair keep it above the chin and fuller on top. Layers soften this face shape and cuts with angles are ideal.