My 45-70 rifles come in a variety of barrel lengths and strengths. From the original Trapdoors and the BC to the little (dare I say it?) Thompson Contender with the youth stock and 16" barrel.
Others have 18", 22" and a custom 26" barrels. All on "our" frames.
My personal preference for my kind of hunting, I don't walk well anymore, is the BC. This rifle's weight and length don't bother me at all because I'm hauling it to one of my stands on the ATV. A set of shooting sticks make stability much easier and the long sight radius is a big plus with old eyes. The thump of a big chunk of lead anchors a deer quick and it's extremely effective on coyotes or groundhogs. Course this is short range shooting, out to about 150 yards or a bit more. For longer range coyotes and hogs I use a varmint rifle with optics.
The little gun worked great when I still was able to run up and down the hills. Weighs just over 4.5 pounds but the ammo weighs up in a hurry as you all know. Recoil with hunting loads was noticeable but manageable with a bit of practice.
Again just my opinion but an 18 to 22 inch barrel would be a good compromise length. Still have plenty of velocity, easy to swing and easy to carry. I'd suggest looking into Magna-Porting or similar work to reduce muzzle jump. It made a difference on my 16" barrel.
Mike