I think Eddie has a point, but if you're thinking of keeping a steelhead based on its dorsal height, here's a thought:
The only time I've been busted for anything was in '84 or '85 when I kept a small, obviously hatchery fish in the Snoqualmie. It was only about 4 lbs and had concrete-worn fins. But I measured the dorsal anyway, with my state issued wallet card,in its natural upright position to confirm it was under the 2" requirement. Along came two WDG enforcement officers with a pair of PLIERS! One yanked the fin up at a full 90 degrees, measured over 2" and gave me a ticket!
My advice is that you first ask the local agents how they define "fully extended." It could save you a lot of grief.