I thought it might be interesting to post a photo chronicle of a betta's growth from egg to adulthood. Every week I'll add a picture set to display at full size for a monitor setting of 72 ppi. This way visitors who haven't raised bettas can view the growth process.
As a reference, the line below is one inch long.
If it looks one inch long on your monitor, you'll be seeing the fish at their actual size. Be warned, young fry are so small that they will appear as little more than specks.
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
After eleven weeks of growth, the amount of week-to-week change is too little to be interesting. From then to seven months of age the betta's body shape stays the same with a very slow increase in size. The fins grow at a glacial rate, eventually developing their full length at seven months.
Footnote from Faith: Final betta size may end up being 1.5 to 2 times as large as the last photo taken by Wayne. Growth vary greatly from betat to betta and from strain to strain and depends on many factors, including type of foods used, frequency of water changes, tank size, general health, betta strain, etc.