My leghorns i got at TSC just layed an egg for the 1st time. They are 18 weeks old. My only concern is that the egg was kinda small, do hens lay smaller eggs when they 1st start laying?
As the hen gets older, her eggs will get bigger. With White Leghorns (the very best egg producing breed of chicken) the eggs should get normal sized very quickly, probably in less than a month. If you have a different color variety of Leghorn you may need to wait a little longer for the eggs to get bigger.