Okay this one is simple. Every skin change results in something smaller. For example a mayflies body is smaller than the nymph it emerged from. A spinner likewise is smaller than a dun, because of another "molt". So remember if you fished a size 16 dun for a hatch, don't even think about fishing a size 14 for the spinner fall. There is one slight exception to his rule and that is emergers. They can be a bit longer than either the nymph/pupa and the adult because the body is lengthen by the shuck they are escaping from.