People who wake up with bed bug bites often assume that the insects causing them such discomfort must be considerably large. This, however, is not the case. Bed bugs in fact are smaller than mosquitos, even though many people find that bed bug bites are the more bothersome of the two.
The average bed bug is no larger than about one quarter of an inch in length, though of course the juveniles of the species will be quite a bit smaller. Bed bugs are also almost completely flat, which can make them difficult to spot. These figures, however, only relate to the size of the adult bed bug that has not yet fed. After feeding from the blood of a host, the bed bug will swell up a little bit, becoming bloated; at this point it can no longer be called flat at all. Its length does not change appreciably as a part of this bloating, however.
More precise measurements of adult bed bugs suggest that they range from 4-5 mm in terms of length. They are usually from 1.5 to 3 mm in width.