think of it as a rear sliding glass door. drop it to the floor with a little bit of rise to it, so you can do trim boards.
if you are a tall person or tall folks in your home. get them to stand against a wall. and measure there eyes, and say top of window 1 foot above that.
if you are going to be making a "bench" per say so you can lay down in front of the window. or sit in front of it. then figure out what is a good height for a bench wood be. with a cushion. so you can either set at bench with back to window or lay down on the bench, with a pillow under your head and look out the window.
your overhang / sofit for your roof may play a big roll and perhaps wanting to drop the window down some.
if ya going to have a small shelf to place little things (drinks, nic nacks, etc...) maybe better to have more waist high.
if ya getting windows it is not that bad to "custom order windows" to fit any size you want. though there are set standards of pre-manufactured windows that might be cheaper, and it may vary some from manufacture to manufacture. and year to year it may change.
if you have dogs/cats. myself dogs, than you might want to raise window a couple feet off floor. so the wild animals that they can be, don't go charging through a window. and more likely a screen in the window. "i keep a childs/pet gate in front of one window. due to it seats right at the floor minus some trim work around the bottom. on other hand. the pups love to sun bath, and catch them laying in front of the given window many times.
does the yard slope up wards? does it slope down wards? if it slops downwards (your up on a hill) place it down, if looking up on a hill place it higher.
if it is a bed room, adjust it a little lower than bed, so you can open windows up and get a breeze going, that actually goes across the bed.
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basement and you need 9 foot ceiling due to tall folks in family. you might want basement a little higher, to deal with say a drop down ceiling. so you run central a/c or heating, along with wires, plumbing etc... and still be able to get to the stuff from the basement.