As I think you already know, the limitations on designing wet scrubbers in general are imposed by five parameters which the most important of them are: Gas Flow Rate, Liquid-to-Gas Ratio and Pressure Drop. Based on allowable limits of these parameters and nature of process designer specifies height to diameter ratios for a wet scrubber. This fact is not only concerned to cyclonic wet scrubber but all scrubbers a well. I suggest study one of the EPA engineering guides on wet scrubbers which is available at following web address:

http://www.epa.state.oh.us/portals/27/engineer/eguides/scrubbers.pdf