I have a Bic Vivace 285 slalom board and want to replace my current fin. It has a US box and i want to be putting a 5.5 and 6.5m/7m sail on it. What would be the best size//type of fin to get?

Try this: www.select-hydrofoils.com/en/selector/

On the RHS table, select option 2 or 3.

In passing, are you sure the Vivace is a US box? I thought Bic was using their own Trim Box on that range of boards.