The Bigger the dredge the bigger the rocks and materials that will be required to roll thru the sluice!
Lets say start with a 2.5" dredge and conventional single surface sluice box. The suction nozzle tip is probably restricted down to say 2.125" so a 2" round rock will roll thru the sluice with relative ease!
Now upsize to a 6", suction nozzle tip restricted to say 5.5" a 5.25 diameter rock is now required to roll thru the sluice. As it makes its way down there will be some quite substantial disturbance to material within the sluice, especially in the low pressure vortex of the riffles, this can migrate gold!
Also the larger size dredges lean more towards effective production machines. So building a longer sluice to cope with higher volume production makes sense!
A hobbyist with a 2.5" dredge getting say a gram or two for a few hours play will not be concerned with a small loss say of 0.1 gram from two grams produced. Where as a professional operator would be worried having say a production loss of 6 grams a day, when producing say 30-40 grams!
In the past Keene has built a number of different style sluice systems in dredges, triple sluice, under & over box, Keene 3 stage system! There is a benefit in the larger sluices say above 4" of screening material so the fine gold recovery can be improved and is not effected by the larger rocks disturbing the high grade values.
As a guide for flair length you would need to look at whats on offer from the major market share Dredge manufacturers! Keene & Proline would be making most of the gold dredges sold world wide at present!
Precision Dredge build high quality production dredges up to 8" & 10". I am not sure if their flairs are longer the the competitors?
If you are building your own dredge you may need to perfect your design by trial and error! This can consume valuable gold getting nozzle time plus use up materials, though i know some still prefer to build their own machine!