Kinda sorta depends on the where and the why, I'd offer...
For example, the "long knives" were used for fighting when an enemy closed with the knife owner before the rifle or gun could be reloaded. They were also used as latch ditch weapons when surprised by an angry bear.
In areas where hunting continued as a profession but the threat of Indian attack was pretty much nil..., long hunting knives were often used to dispatch wounded bear. Meshach Browning documents in the first decade of the 19th century, in the panhandle of Maryland, dispatching several bears with his knife.
Then you have German Jaegers with rifles that did not carry a bayonet. The Jaegers normally carried a hunting sword to dispatch wounded game to save on powder and lead, and when they served in an army, these served as a backup due to the lack of a bayonet.
So then....how long is too long?
A lot of historic, preserved Bowie knives, made for and actually used for fighting, have blades from 7-12 inches. On the other hand, in the 18th century on the frontier, a fellow probably used a common butcher knife, not a custom made fighting knife, for his "long knife", and so perhaps the blade was from 7-9 inches long?
Like Black Hand, I have found that a smaller sheath knife and a small tomahawk do much more for me around camp, and when dressing game, than a full sized butcher. When I carry a full sized butcher knife, it's because I'm portraying a hunter at a venue where I'm talking to the public about history, and I would've had one on me for self defense, and not for utility purposes.
LD