what size of egg to incubate

[adorable]

My hens started to lay about a month ago.How big should the eggs get before i can incubate them. ?

 

[My Little Sister's Farm]

I'd start when ever. It won't retard the chick to hatch from a smaller egg. It will only get as big as the egg to hatch, but as soon as it's out it will grow to it's genetic potential assuming nutrition is adequate. If you want to start incubating, I say go for it.