Bed bug size depends on the age of the bed bug. The bed bug life cycle consists of three main stages: egg, nymph and adult.
Bed bug eggs are about 1mm long (about the size of a poppy seed) which makes them very difficult to see when they are single. They often occur in clusters though which are possible to observe.
Bed bug nymphs move through 5 stages of development through the molting process.
Depending on how far along a nymph is, it will be somewhere between the size of a bed bug egg and an adult bed bug.
Adult bed bugs are about 3/8 inch long. When engorged from a blood meal, they may appear a bit larger.