The first question about the size of the hatching eggs you are setting ~ usually pullets lay small eggs when they first start laying. Depending on the recipe/feed ration formula ~ those eggs will increase in size, directly related to the feed ration. Mine transition to medium in 2 weeks, with other feed rations, it has taken 2 months.
You "can" set pullet, small eggs. It will result in a small chick. How big those chicks grow will depend on the nutrition of the hen 6-8 weeks before those eggs were collected for and also quality of the chick star.ter Also, genetics impact the health and size that the chick eventually attains.
It's best to wait until the eggs are a medium size. My feed ration is formulated to produce medium eggs which are the ideal size for hatching. Large eggs tend to have less fertility than medium eggs.
For best results, feed your mature birds a breeder ration 6-8 weeks before collecting eggs for hatching. Place those eggs in an egg carton/cartons and put those in a box which is propped up on one end. Rotate the box in the same direction, a quarter of a turn twice a day. The ideal temperature in the room the eggs are being stored in is 55 degrees F. If this temp is not possible with room temps, some breeders store their eggs in a wine cooler. Room temp should not be over 76 degrees F.
Ideal length of time to hold eggs is 11 days. I have, however, held eggs for as long as 18 days. The longer hold period usually results in lower fertility and/ or early deaths of some of the embryos.
Fresh pullets who have eggs transitioning to medium size will give you your best hatches. With good genetics, good feed ration, you will get 90+% fertility and about the same for hatch rate.
If you choose to use a small incubator, you can set eggs when you see a hen go broody, and transfer the eggs to her/under her when they are near the hatching date. This process increases the hatch being successful since is the egg are incubated without the competition of other hens in the nest area and utilizes her as your brooder, far more efficiently than any of us could be.
The above information is what I have learned over the past 7 years as a breeder. I am happy to share any information ~ I've already made most mistakes that can be made. No need for anyone else to repeat them.