Bank walker -
Rather than measure fish with tape a trick that I have used is to use the rod to measure the fish for a quick and dirty estimate of the size of a fish.

For example my favorite drift rod is 39 inches from the butt of the rod and the first guide. My favorite fly rod is 34 inches from the butt to the stripping guide, 40 inches from bottom of the cork to the second guide. It takes just a second to lay the rod beside the fish while it is still in the water and I then have good idea of the quality of the fish. For example a "Dolly" that comes within a inch or so of the stripping guide or steelhead the stretches form the corks to the second guide is darn nice fish and that is enough for me.

Note: the rod will measure a little shorter than a tape stretched over the curved body a fish.

Tight lines
Smalma