In my days of doing juried shows I sold a lot of rings, each fabricated one at a time (bands with soldered on bezel cups), and found that the top of the bell curve was around 7.5 to 8.5. But this is not ironclad, there are a couple of considerations which can move this up or down: First, gender - less than 10% of my sales were for men, they average 9 - 11, and range over a wider number of sizes than do women. Second, age - knuckles expand with age, and if the bulk of your sales are to 50+ women the curve might top out at 8 to 9.5. I began selling my rings in a college town a block off campus, ring fingers clumped around 6 to 7.5, but when I began doing shows this spread moved up a size and a half as the age of patrons increased. I’d sit behind 12 ring trays with at least 800 rings from size 3.5 to 13.5. The best selling sizes (6.5 to 9) would have nearly a dozen columns which were about half my sales; larger and smaller sizes made up the other half. And to paraphrase Wayne Gretzky, 100% of the rings you don’t make up in less popular sizes don’t sell. I had a couple dozen popular styles, and a ton of popular stones in dozens of materials, so bands that sold well I tried to keep one in every size from 5.5 to 10.5. For about 15 years I toted around a pegged board with plain “cheapie” rings, just plain silver bands, 8 G half round (size 2 to 8), 6 G half round (size 4 to 13), 6mm low dome, 8mm low dome, 10 mm low dome, and rolling rings (3 interlocking 8 G). I went through a few employees fabricating them and keeping tumblers going. Hope this helps. 18" necklaces sold in about the same number as 16" and 20" combined.