If you're a 8.5/women, then you're roughly a 7.5/men. Based on that, I would try the large. I can't guarantee it, but I think you will likely be able to pull the straps on the open heel enough to tighten securely around your foot. On the other hand, if you get the small, and you can't get your foot in, you're out of luck. And the top end where your toes go is small; I'm a 9.5 men, and the fit there is very snug.
I hope this helps. BTW, I used these many times just last week in Cozumel. The short fins mean that you are going to have to kick a lot more, but the payoff of being able to easily fit these in my carry-on more than made up for that. I hope you're off to somewhere equally beautiful . . .