When I bought my first ring sling, I waffled on the size for a straight month. I was used to clothing sizes, where being slightly off can make a shirt or pants completely unusable. Luckily, ring sling sizing is a little more flexible, and many people can use multiple sizes safely.
Any ring sling should have at minimum 12 inches of fabric hanging from the rings for security. If you go shorter, you also run the risk of your sling coming loose from the rings as you loosen.
As to how long it must be, there’s no hard and fast rule, which makes ring slings so easy to share between partners of different sizes! Some people prefer it shorter, to the hip, and others prefer it long and flowing. My personal sweet spot is the upper or mid-thigh, so the tail will be long enough to use as a nursing cover. I find that if I go down to shin-length, any items I store in the pocket become a bit of a pendulum and hit my leg.
At Maya Wrap, we offer 4 sizes, from small to extra-large. Small is 67 inches, medium is 73 inches, large is 79 inches, and extra-large is 85 inches. An easy way to see just how a certain size will fit you is to take a measuring tape or a piece of string, and measure from the armpit over the shoulder, around your body, back to the armpit again, and then down.