For the young ones I use zippies for the babies and clips for the adults, and cut the zippies before they get to tight and add another one of the same color.
I go by the last toe to see if i can slip the band off or not, if i cannot and there is still room to move then i leave it. Test it with the toe, i leave enough room to get a pair scissors underneath them.
For the bands I use med sized for my adult guineas and chickens, but need to get a pack of large for my larger legged girls. But for now I keep a good amount of wiggle room on them, and they are great if their leg gets caught it snaps the band open and falls off