The reason one waits with pullet eggs to increase in size is that the tiny little pullet eggs simply allow no room for normal chick development. I would personally wait until the eggs nestle nicely in a large egg carton. They don't need to be jumbo or anything, but nice large is just preferred. It's a lot of work and dedication to fuss with eggs for 21 days only to be sorely disappointed.
They normally increase in size in their second or third month of laying. At 18 months the hens will moult and then once they lay again, they will lay somewhat fewer eggs, but my goodness, they sometimes REALLY increase in size. This is why breeders, true breeders, not just propagators or hatchers, prefer to hatch from older hens. The egg is larger and the breeder gets to see the hen after a moult. What is her feathering and coloring look like now/? For certain, we don't use females less than 11 months old in our breeding programs.