A bed bug is a nymph through 5 stages in the bed bug life cycle. So the size of the bed bug nymph depends on which stage the bed bug as it. Here is roughly how long the bed bug will be at each stage.
- 1st Nymphal Instar – 1.5 mm
- 2nd Nymphal Instar – 2 mm
- 3rd Nymphal Instar – 2.5 mm
- 4th Nymphal Instar – 3 mm
- 5th Nymphal Instar – 4.5mm
You can get more information on bed bug size by reading Bed Bug Size and Life Cycle.