Lonerider here is how I set mine up everytime and all the Flows I've owned and has worked for me everytime
- Loosen power strap ratchets and fold down high back
- step into binding and close highback
- tighten powerstrap closest to ankle then the toe section
- lock the ratchets I think you still have the locks on the 13's, Pull up on the ratchet and then lock them this will keep the ratchets from tightening on you.
I would stick with larges as this will still give you some room if you have a boot with a larger footprint or bulky type boot, IMO