Leathers will generally come in 48, 56, or 60 inch lengths.
48" is sometimes labeled as Children's. Check to see if there is a weight limit...if they are cheap leather, they may snap over a jump with a heavier rider.
56" is standard, Adult.
60" would be for dressage or really tall people with gorgeous long legs.
I'm about 5'5" (with short legs) and I always buy 48" leathers. I'm usually on hole 6-8 (out of 12, one inch apart). This gives me enough extra length to fit in the keeper, but not so much length that it flaps around on my horse's side/flank.
Things to keep in mind:
Non-lined leathers will stretch with use, adding about an inch or two over time (remember to switch the sides of the saddle you put them on or the left one used for mounting will gradually be longer than the right).
The height of your irons also affect stirrup length. If you have really tall irons, you'll be fine with shorter leathers.
If you get too long of leathers you'll have to either wrap them around your irons or punch extra holes. Wrapping them causes extra wear and is sometimes uncomfortable or twists weird. Punching extra holes reduces the strength and integrity of the leather.