These are data about mouth size of northern pike from Cecil Baird in his Taxidermy shop.
"32 inch northern: vertical gape 3 inches, horizontal gape 2 3/4's.
42 inch northern: vertical gape 4 1/4 inches, horizontal gape 4 inches.
These are of mounted fish and may be on the conservative side. I have two muskies in the freezer in the 45 inch range but didn't get to them today.
Of interest may be a 48 inch northern gill netted by the INDNR years ago while collecting broodstock walleye from Clear Lake, Fremont IN was brought back live to a holding tank and spit up a 20 inch rainbow trout."
I think a 20" long rainbow trout probably has a similar or very close to body depth of a 20" LMB,
