A longer sluice will keep the gold longer, the hydralic miners of old had 100 yard sluices called long toms which they cleaned out weekly, they still blew most of the fines through because it wasn't classified material. Maybe the first sluice runs 1 inch minus material which dumps into a second sluice covered with 1/4 inch or 1/8 inch perforated sheet suspended 1/2 inch over the rifles, Rocks just slide right over perforated sheet (punch plate). You might want to check out Keene engineering, they've got this new black rubber rifled matting to catch the fine stuff ( $8 per 1x30inch strip). I bought a 10 inch wide strip to do the exact same thing (as above), I hav'nt put it together yet but I'm hoping it works on lake Superior black sand.
Anyway running the finer material through a second sluice ( or train of sluices) would be the only reason to go longer than 4 feet on a 10 to 12inch wide sluice. Good luck, Bill