My personal experience has caused complete enchantment with these glorious beauties. For example, the dahlias planted at the front of my house bushed out and bloomed profusely from July until the first frost. The dahlia at the side did not fill out as much but the height achieved was quite amazing. One day in September I noticed a gigantic bloom at the top of the plant and realized I had to extend my arm fully to touch the flower. This plant reaches the window ledge of my house.
To obtain the best floral display choose a site which receives at least a few hours of sunshine on a bright day. Do not plant under trees nor in soil which gets water-logged. Medium loam is ideal, but most soils are satisfactory. In autumn dig in plenty of organic matter such as compost. Rake in Bone Meal after digging.
Some catalogues use words to describe the average height of a variety grown under good conditions. Do remember that the stated height is only an average and the actual height achieved by a plant in your garden will depend on the location, weather and cultural conditions.
Some catalogues use words to describe the bloom diameters of well-grown Decorative and Cactus varieties. The key below will guide you in making your choice.