The best length for you is dependent on a few things.
Your hair texture
your hair density
your face shape
your preference
Hair Texture
Most people have some natural wave to their hair. If you have embraced your natural wave/curls or are working toward embracing it then that will effect what length looks best and works best with your texture.
Often wavy/curly hair may be weighed down if it is too long which can make softer waves almost non existent. So some of your length choices may be dictated by what makes your texture look best.
If you have tight springy curls you may loose a foot of length with shrinkage so longer will work better for you.
for most curly/wavy hair layers are your friend, this will keep you from the dreaded triangle head.
Hair Density
If you have low density hair or fine hair you as your hair grows longer the ends look sparse and see through. For some this can make the hair look unhealthy so opting for ends that are more blunt will make your hair look fuller, thicker, and more healthy.
In contrast if you have a ton of hair, it may not have any movement or texture because it is thick and heavy so opting for a more textured and layered look with choppy ends will lighten everything up and make it more manageable.
Face Shape
We are taught in Cosmetology school that the goal with hair and makeup is to create the ideal face shape, oval. Personally, I think this is out dated, we should be celebrating differences not trying to conform to some standard of ideal beauty. So I won't be giving you the advice based on what we were taught in school.
Hair can be used to accentuate features that we like, and take away from features that we don't want to show off. For example, if you don't like your ears to show you may want to opt for a style that is long enough to cover them and provides shaping that doesn't make you want to tuck your hair behind your ears, maybe that's bangs or layers in the front, or maybe that is hair that is all one length, because it depends on what tends to make you tuck your hair behind your ears. But maybe you love when your ears show, you love wearing earrings that you want to show off. Then you may like a short cut, that shows them off all the time, or a shag where there is lots of texture around the ear so it shows often.
If you like the shape of your face in general, then something with lots of layers around your face and above your shoulders may feel like the right fit. But if you feel like your face is too round or you don't like what's happening in the chin area less layers in the front, and a bit longer than your shoulders may be better.
Preference
Truly, this encompasses all of the above, but also we need to take into account what works with your lifestyle. Are you a nurse? You probably want your hair up, or short enough that you can wash it quickly in a sink in case work gets messy. Or your a new mom, and baby just got to the age where they're grabbing anything within reach during feedings. Might be time for a shorter cut if your sick of the daily mom bun.