It depends on the wick arrangement of the atomizer. For most RDAs I find a large diameter coil loosely wicked provides great flavourful vapour. On most RTAs however - without considering the width limitations of the build deck - there's a need to balance the coil diameter with the openings to the reservoir for the wick. Too large a diameter results in excess cotton choking the flow from tank to coil - dry hits on a mec or a 'thin' vape in TC. I use 3-3.5mm in RDAs and 2-2.5mm in RTAs. Others may get different mileage, but it works for me.