Way before then.. their fins continue to grow until around 7 months, and they tend to be sold around 2-3 months at the stores.
This is about as accurate as one can get when it comes to measurements. This is average, have to keep in mind water conditions and feeding habits/food do make differences in their sizes.
Betta egg: 0.03 inches in diameter
One day old fry: 0.1 inch long (The tail doesn't show at this scale)
One week old: 0.2 inches long.
Two weeks old: 0.25 inches long. (Starting to show dorsal fins.)
Three weeks old: 0.34 inches long
Four weeks old: 0.45 inches long on average.
Five weeks old: 0.6 inches long. Size ranges from .5 to .75 inches.
Six weeks old: 0.85 inches long. The largest are over an inch.
Seven weeks old: 1.1 inches long.
Eight weeks old: 1.3 inches long.
Nine weeks old: 1.55 inches long.
Ten weeks old: 1.7 inches long
Eleven weeks old: 1.9 inches long