Best ammo size?

My recent involvement with sling shooting and 40 years of woodworking have culminated in a few creations from leftovers as well , a good part of the fun actually. My limited experience says light bands and light ammo to start as others have said. 3/8 steel is moderate heavy but seems like even with lighter bands you get the feel of trajectory and they're easier to follow than 3/8 clay. With mid-weight bands 3/8 steel works well and a good analog for it is 1/2" marbles if you are going to lose it. I would like to run across some 3/8 marbles as that seems like it would be a good alternate to 5/16 steel.

Fellow woodworker here too. I think it depends what type of shooting you're doing ... if you're shooting outdoors primarily, clay balls are a good bet; if you're shooting indoors with a catch box at paper or spinners, 5/16" or 3/8" steel are most common. I'm a paper guy and found that shooting paper plates is cheap (can get 300 of them for $5 at the super market) and they cut much cleaner holes than printer paper, as the paper is thicker. As a woodworker, you'll appreciate - I used forstner bits to make circle tracing templates for 4 & 6 cm - works great & allows making targets ... quickly. I like 5/16" as it cuts a cleaner hole than 3/8" at least at target velocities, and is a bit easier on the fingers unless a person is skilled enough to hold in front of the pouch. Lately, I have shot my all time best with 7mm (9/32" steel) ... it's right in between 1/4" and 5/16" - easy to find online and I'm really liking it for paper - hits hard and flies dead straight at 10m with light band setup.