Looking at most things in the fishing world the larger the number the smaller it is. Duo locks are different for some reason. A size 0 is 13 mm long, max weight 26 lbs. A size 8 is 48 mm long with a 220 lb weight limit.
Here's a bit of a chart.
100pcs #0-#8 Fishing Duo-lock Snap Stainless Steel Nice Snaps Tackle Connector | eBay