The heigh of your curtains can change the feel of your whole space. In general, closer to the ceiling is "better" than closer to the window. Hanging your curtain up higher can make your windows seem bigger and in turn make your ceilings feel taller - YAY!
You can always customize your curtain length, but standard curtain lengths come in 63, 84,
96, 108, and 120 inches. Another thing to think about is how you want the curtains to hang. I tend to be a floor-length kinda girl because I like the dramatic effect. But there's also a "puddle" length and an "apron" length. My vote, if you have a full wall to work with is almost always floor length. Puddles tend to get sucked up by vacuums and collect a buncha dust bunnies. An apron-length curtain (for me) feels abrupt. With that being said apron length curtains work great over countertops, tables, dog beds, etc.
Of course, this isn't always possible. If you have not so tall ceilings or super tall windows, hanging a curtain rod 4-6" above the top of the frame may not work. My rule here is to split the difference. If you can put a curtain rod 4" above the window frame but it's basically in the ceiling.. no bueno. My recommendation is to measure the distance between the top of the window frame and the ceiling and split the difference!
It also maybe won't work if you live in an apartment and can't put holes in the walls.. been there! These things, Kwik Hang are game-changers for you apartment dwellers! You can get away without putting the screw into the wall too - just make sure to choose some lightweight curtains! If this is you, I would definitely then recommend going floor length if you can!