That's a complicated question! Riffle height is one part an equation that also includes, style of riffle, riffle spacing, riffle angle, sluice box angle, depth of water, speed of water, type of material being run, size of material being run.
you want your riffles to be the height of the material your running.. !/2 inch classification = 1/2 inch riffles. around 1 1/2 to 2 inches apart. If your going to have slow flow and only fine gold classify more and go smaller.