With asparagus, bigger is not necessarily better. It's really a matter of preference: thin spears are more tender and sweet, whereas the more mature thicker ones have a stronger taste. Regardless of your aim, seek out bunches that have similarly-sized spears to ensure even cooking time. Then, inspect your selection from top to bottom: the tips should be closed and compact, the stalks should be straight and firm, and the base of each stalk should be moist rather than "woody."