Tulips originated in the mountains of Kazakhstan. Years of breeding by tulip growers, primarily in the Netherlands, have yielded hundreds of tulip varieties and cultivars. Commercial growers grade tulip bulbs by size, and most bulb vendors refer to the largest category--bulbs 12 centimeters in diameter or greater--as "top size." However, nomenclature is not firmly established across the industry, and different terms are used for the largest bulbs intended for different purposes.
Top Size Bulbs
When purchasing tulip bulbs, bigger really is better. According to spring bulb experts at Gardener's Supply Company, bigger bulbs planted in the fall mean bigger, more reliable blooms in the spring. Bulbs are sorted by size, and reputable bulb dealers state the size of the bulb as centimeters in circumference. Bulbs referred to as "top size" should be 12 centimeters or more in circumference--although, as the University of Arkansas Extension points out, the largest bulbs are likely to be more expensive than smaller, discount bulbs.
Tulip bulb sizes also vary depending on the variety and cultivar; the best quality species tulips or dwarf-flowering tulips may have bulbs considerably smaller than 12 centimeters in circumference. Leading bulb suppliers in the United States, like Breck's and vonBourgondian, also sell "Deluxe" or "Mammoth" tulip bulb collections, guaranteed to be a minimum of 14 centimeters in circumference. These enormous bulbs are certain to sprout big, bold blooms the following spring with proper planting and overwintering conditions, but they will carry a premium price as well.
The largest tulip bulbs will present the largest blooms on the sturdiest stems the spring after they are planted--but they may not return to bloom in subsequent years. Carefully planted, fertilized, watered and tended, most showy tulips will return for 2 or 3 years, but field trials at the University of Arkansas demonstrated that only one cultivar (the red tulip "Oxford") kept returning consistently after 3 years. Tulip varieties that naturalize well are not necessarily the varieties with the largest bulbs, and a mix of bulb sizes means that smaller bulbs will develop in the ground for a year or two before blooming, stretching the number of effective blooming seasons from your planting for another several years. Check for bulbs sold by reputable dealers marked for as good for naturalizing, rather than on the size of the bulb sold, for the most enduring tulip displays.
Bulbs for Forcing
The largest tulip bulbs for early forcing are marked and sold either as "for forcing" or as "exhibition size," according to the Tulsa Master Gardeners program. However, it is hard to specify exactly how large these bulbs will be by measurement, because they vary dependent on the variety and cultivar you are seeking to grow. Try to obtain the largest bulbs available for your desired tulip type for forcing, and make sure the bulbs look plump, not dried-out or moldy.