Medium.
I can use mine for half-ups and ponytails no problem, so you should be fine using it with buns etc. on your fine hair for quite some time. And as for getting a lot of use out of it as your hair grows...chances are you will never need a bigger size. I'm at tailbone length, and a medium holds my buns just fine. It is just a matter of learning how to use it.
Though...it is on the smaller side now. Thankfully, it is time to cut back to bsl again.
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