Sizing a static mixer is often best handled by the simple approach.  In most cases, the diameter of the mixer should be the same size (diameter) as the pipe leading to and from the mixer.  The velocities needed for good mixing in a static mixer are similar to the linear design velocities through a pipe.  The velocities should be high enough (2 to 3 m/s) to limit the cost of the piping and low enough to prevent excessive pressure losses.  Pressure loss through the static mixer may be 5 to 20 times the pressure drop through an equivalent length of open pipe.  However, the static mixer for turbulent mixing is short and the added pressure drop is usually not a problem for the pump.
 
With the water flow rates and anticipated velocities, flow should be fully turbulent making rapid mixing quite efficient.  Twisted-element or other high-flow, low-pressure drop mixers should be used.  High-pressure drop, structured-element mixers are not needed for turbulent mixing.  Some static mixers are designed specifically for turbulent mixing.  As few as two or four elements may be needed for turbulent mixing.
 
Ideal installation of the static mixer should be with 10 pipe diameters of straight pipe upstream of the mixer and 10 to 20 pipe diameters of straight pipe downstream.  Some turbulent mixing is accomplished after the flow leaves the mixer.
 
The injection point ahead of the mixer should be less than a pipe diameter upstream and have multiple (at least 2 or 4) injection points across the pipe diameter.  Those points should be located in rough proportion to the cross-sectional area of the pipe.

The answers by this expert are based on the best available interpretation of the information provided.  The consequences of the application of this information are the responsibility of the user.  If clarification is needed, please submit a further question.