For me, it typically depends on the species, although I almost never carry measuring devices of any sort. Generally, bigger species I tend to measure in pounds. For most freshwater, I would measure in inches - like mentioned, a 22" bass helps me understand size better than saying 4.5 pounds. I also don't care if it's an 8 oz brook trout versus a 10 oz brook trout. With most saltwater, though, I prefer to measure in pounds. When I'm bluefin tuna fishing, inches only matter so you know whether it's legal commercial size, but inches don't matter when I go to sell it. You want the 95" fish that weighs 400 lbs, not the 100" fish that weight 380. Similarly, I have no concept of length when it comes to trophy redfish. Sailfish and marlin are the exception, though. I only estimate weight based on length and girth.