Thein Separator inlet elbow diameter

I intend to build a Thein separator based on a 5 gallon paint pail feeding a 8 gallon Shop vac . All the pictures and docs I have seen shows the inlet elbow to be 4 inches . This seems excessively large for a 5 gallon pail with only 12 inches dia across the top lip of the pail thus leaving very little separation between inlet and outlet pipe. Can I reduce the inlet elbow to maybe 3 inches and not affect performance perhaps due to too high velocity of the input stream ? Tks

(85f)

Thein Separator inlet elbow diameter