Bank walker -
High teens sounds about right. Most 20# fish are around 38 or 38.5 inches in length. Have seen fish as short as 36 inches made 20# and one 42 inch fish that did not. Some variation from river to river and year to year.

Not sure that I understand this need to take pictures and measure or weight every fish. The idea in releasing fish is to maximum its change of survival. For that fish of a lifetime I strongly recommend that if in a boat taking it to shore, slide it into the shallows (2 to 4 inches of water). There a quick picture and/or length can be taken without ever removing the fish from the water. Notice no nets!

An observation: It appears that CnR works the best for large fish as the number of 20# caught (% of the run) is much larger than in the old days of kill and weight them! LOL!

Tight lines
Smalma