How to define the time step size using CFL number

Dear Experts 

Greetings 

I am working on the transient dam break multiphase flow. For faster and accurate simulation I need to set the optimum mesh size and time step. how to decide the time step using CFL number. Suppose I have to simulate the transient flow of with maximum front velocity 2m/s. I could not understand what should be the value of maximum velocity fector and mesh scale. Further it is transient turbulent flow whose CFL number must be less than 1. Plz guide to choose correct time step in my case.

Thanks in Anticiaption