Imo it should be based on feel, not necessarily shoe size. My shoes are size 13 and I still love my friends tiny ass Baker board to death. I want to say its smaller than 7.5, but it might not be. I know my darkstar was a 7.5 and I could tell a difference when riding his, but I still think his was shorter.
EDIT:
Yeah it was a 7.25 x 28.75.