Had to find some pics with a scale in them.
If I remember right, this horn held just about 3/4-7/8# of powder:

This is my main horn. I've used in matches, carried it on jaunts across (literally) thousands of miles, hunted with it from the Arctic Circle to south Texas. Capacity is just a hair under 2#:

It originally made it in a class I got strong-armed into conducting at a rendezvous back in the late '80s. As originally made it held just over 2#. After a canoe wreck left it underwater for a week or so ~2005, the base plug cracked. I wound up shortening the horn at the base and fitting a new plug (you can see where the scrimshaw got "chopped" in a couple places).
I do tend to use loads that are heavier than others, but I also shoot a lot--and this is also the horn I use when teaching "Intro to flintlocks" where we sometimes shoot 100 rounds. At the most-recent match I shot--what we are calling the "First (Possibly-annual-but-maybe-not) Fall Invitational"* (not to be confused with the actual "Spring Annual Invitational"*), I shot two rifles (.50 percussion and .51 flint). One of my shooting partners doesn't have a caplock, so he borrowed my Leman for the percussion aggregate. Three aggregates and the side-matches totaled ~120 shots, burning almost 1.5# of powder.
I also took my Colonial as a spare/loaner. It got loaned to several people who wanted to try it out. Someone sponsored a "Kibler match"--you had to shoot one of Kibler's rifles--and there went another 20 rounds for me, one of my shooting partners, and a couple of shooters who borrowed it. There went another 1/2 pound plus.
This was actually only the second time I've had to fill this horn at a match.
*"Invitational" is a fuzzy term. The email invite last spring said "bring a friend, a newbie, your daughter's boyfriend, or some guy off the street".