One of the first dahlias I ever got was purchased as a potted plant, and was only about 12" x12" at full bloom. I saved the tubers, planted them the following year, and the plants were more than twice as big, about 2.5 ft, tall and wide. Apparently there are chemicals that nursery's sometimes use to stunt the growth on plants and keep them small for containers.
So……ask them at the nursery and try to find out more about what you are purchasing.
