Got my first egg yesterday at 18 weeks on the nose. I new they would be smaller but was surprised at just how small it was, like bantam size. Have no idea who layed it but I am sure that it was one of my red sexlinks.
Usually by 7-8 weeks they will be laying their normal size. Your chook laid a larger egg than my youngest started with...she laid "peewee" size in mid February, and now has been laying "Large!!!" She is also the one that my 6 new chicks hatched from! Soooo you are on your way!
Congrats on the eggs! They will gradually get bigger over time. Sometimes the change is so gradual that you don't even notice it. Then you may get another jump in egg size after they go through their first adult molt.