To me, northerns are the "slipperiest" fish to guess weights on. They can be long and skinny, long and massive, squatty fat, etc. I have no idea how much that one weighed, but guessing on northerns is a shot in the dark.
That's a nice, thick fish, Nick. I'd lean more towards seven pounds, but one thing you can be certain of, Nick, is that when it comes to the size and length of fish, most of us are guessing. I've got rulers attached to the bottom of my canoe, but I don't even trust those. My canoe isn't flat. Fish sure aren't flat. And unflat, flopping fish are measured on a concave surface in a literal second.
P.S. - It's good that you didn't get the girth. That's a measurement I'd trust even less on Mr. Mucous. It's better to return the fish to the water as quickly as possible to thank him for the thrill of touching something so wild and lovely.