They were five days old yesterday. That's probably too early but I like to try to band them around day 5 or 6 and if they slip off I'll just try again. They seemed to fit pretty well though and o wouldn't be surprised if I didn't have to do it again.
I try to band at 5-6 days also. I was sure the band would come off the 110 gram baby but it stayed!
The one in the front is definitely slightly bigger but theres also some unintentional forces perspective in that picture. They both had full crops when I took them out yesterday and the parents are definitely feeding both of them reasonably well. If the gap between the two starts to increase and the smaller one falls behind I'll intervene but I don't think I'll have to

Good to hear ! You know what you are doing.
They've been doing a good job keeping them warm, I have a small rabbit feeder type thing installed on the wall across from the nest box so mom and dad don't have to go into the aviary to eat which helps I think.