Ring Slings....size???

[Charliemarina]

ok so iv been using the tomy baby sling but i will admit its killing my back and LO doesnt seem to like it too much so iv decided to go for a ring sling as they seem the easiest and most appealing to me, but im confused, iv never used one before and have been looking around and they seem to come in sizes
is the size based on my size or is it based on how long i want to use it for and the bigger one i buy the longer baby will fit into it??

sorry if i sound stupid also if anyone has a good site for price and patterns pls could u post them

thanks in advance xx