How do I know what size jump rope to buy (7.5 or 8.5ft) if I am 5'4"?

Question

I am buying it online so I cannot try it out.

Answer 1

If you can, try it in the store before buying it, and see which size you are most comfortable with. If you are a serious customer, the store should not object to this

Answer 2

Take a string, stand on it, and measure how long it is when you stretch it from armpit to feet to other armpit. That's how long a jump rope should be, so buy the size closest.