I've read that birds as small as canaries can be chipped but I really wouldn't feel right about chipping something that small. I would have reservations about doing something conure sized, I didn't like my dogs being chipped either!Despite that I want my green cheeked done because bands can be cut off if a birds stolen or someone finding him decides they want to keep or sell him.
On the other hand vets don't automatically scan birds for microchips so I suppose if anyone else somehow got hold of him with dishonest intentions nobody would know anyway even if he did end up with someone who cared for him and took him to the vet for check-ups

I also think its unlikely he would be stolen as our flats are really secure and we are second floor and he is unlikely to escape because I'm pretty OCD about windows + I'm looking into getting some sort of mesh guards on them anyway.
It's hard, I still want him done just in case but on the other hand I'm not sure the risks are worth putting his tiny body under the procedure.
Looking forward to seeing other peoples thoughts on this.