The answer is it depends on the size of your foot and the booties you are wearing.
I take a 9.5 shoe and I wear Seasoft TI Stealth 5 mm booties (size large). My Atomic Liquid Blue split fins are Extra Large.
My wife's shoe size is between 8.5 and 9.0. She wears medium SeaSoft TI Stealth 5 mm booties and her Atomic Reef Reds are size Medium.
My 13 year old daughter wears size 9 shoes. Her SeaSoft TI Stealth 5 mm booties are size medium. Her Atomic Smoke on Water split fins (regular straps) are size Large (I put the spring straps on my Liquid Blue fins).
You would be safer trying on the fins at a local LDS with the booties you intend to wear with the fins (if possible).
I bought my fins from the LDS since they take good care of me I paid extra and bought them from the LDS.
If that is not possible order two sizes of the same fins from LeisurePro and send back the fis that don't fit.
I pasted their policy below for your convenience.
PFF (Pressure Free Fit) Guaranty Program Information
Your home is our changing room. Try it on and walk it around. Maybe you'll find that the fit isn't quite right, just whistle and we'll come running.
We'll pick up your item and bring you another size - without additional shipping costs. And at Leisure Pro, we'll keep going back to the cyber shelves until you are completely satisfied and perfectly comfortable. Our PFF Guarantee makes shopping for Suits, BCDs, Fins and other dive gear simply easy. Go ahead and try it on with confidence.
Save the pressure for your air tanks.
For more information call us at 888-805-3600