NAC Hovie SUP Race Elite Race Boards Size 12'6" or 14"

We are looking to condense the money class in the NAC Hovie SUP festival Race next April 16th to one board size. Doing this we can offer a larger cash prize with the signing of a top sponsor. It will run a 6 mile race inside the harbor for the cash. We are still 50/50 on doing a distance race as well... What i need to kind out, racing 6 miles, what would the ideal board size be? 12'6" or 14'
Your comments are welcomed.