Now that you’ve cast on, and presumable done a bit of ribbing for the cuff, you have a choice. Either continue with ribbing all the way down the leg, or just do an inch or so of ribbing and finish the leg in some other stitch pattern. Your choice!
The only thing you want to remember is not to make the leg too long. It can be as short as you like, but calf anatomy limits the leg. Basically, you don’t want the leg to be long enough to need calf shaping, unless you plan to go all the way to the knee, in which case, it’s technically a stocking, not a sock. Just make sure that your sock-leg won’t go higher than the base of the gastrocnemius muscle (aka calf muscle – but gastrocnemius is fun to say). For most legs, that means about 8 inches max – maybe 9 inches for a tall guy or 6 for a small woman.
Anyhoo, just make the leg as long as you want it! Easy!