Q: What size egg is best for hatching?
A: There is no special size that is best across all breeds. Think about it: some breeds lay very large eggs because the chicks are comparatively large; some bantam breeds may have tiny eggs, and the chicks that hatch from those eggs are tiny. Hens of vastly different sizes lay different sizes of eggs.
In fact, there are some large fowl breeds that lay small eggs, too... so the size of the egg you want to use for hatching depends on the breed. If that's the case, how do you know which fertile eggs are best to hatch?
Choose eggs that are normal in size AND shape for the breed. For instance, these eggs are all normal in size for the breeds that laid them. Look at the size of the Leghorn egg on the left!
Choose eggs that have regular shells (meaning no bumps, pimples, thin shells, ridges or other problems). This egg has a big calcified bump on the shell and will probably not be the best choice for hatching (although it is fine for eating).
Think about it: the chicks have to peck their way out. They should not encounter a hard spot that they can't get through.
What you ALSO don't want to do is use the small eggs from young pullets who have just begun to lay. The reason is that it can take a young hen a few months to begin laying eggs that are full size for her breed. Her eggs will start out smaller, because they have to pass through her reproductive tract, and it will take some time to become elastic enough to pass a full sized egg--full sized for her breed. If you use an egg from a young pullet that is too small in size, the chick may not have enough room to grow and develop properly. It may not hatch, or if it does hatch, the chick could be weak.
So, when it comes to choosing eggs to use for hatching, you always want to use eggs that are regular in size. Just what "regular" is, though, will vary by breed.