The American Dahlia Society recognizes 16 different groups or types of flower forms and 15 different color categories (see list at the end of this article). It tends to get overly complicated and you don't need to memorize them all in order to grow beautiful plants. A few minutes with a catalog will make it all more clear.

Dahlia flowers range from less than 2 inches to more than 10 inches in diameter. Some flower forms have five different sizes.

To grow very large (sometimes called "dinner plate") blossoms, gardeners must limit the number of flowers that develop to as few as four. If your goal is a large number of flowers per plant, you will want to grow varieties with blooms less than 6 inches in diameter.