as St George noted it's been discussed in detail. The most common knives used by Indians, Mountain Men, and Scouts up through the 1870's were of two basic trade knife types:
1) pre-1850 there was the scalper made mostly by English Sheffield makers such as John Wilson here's an example of an 1820's era scalper - this is from the Museum of the Fur Trades "Fur Trade Cutlery Sketch Book"
http://www.furtrade.org/store/books?product_id=126 - IMO a must have for anyone interested in historic knves:
These typically had half tangs and thin blades in the 6-8" range
The other style that became the most popular after 1830 (albeit they began being made in the 1770's) was the so-called butcher with the most common blade lengths being 7-10" length and again the blades were thin by modern standards usually no more than 1/8 thick. Thousands were imported from England by again such makers as Wilson, but by the 1840's the Green River knives made by Russell in the USA were making a large dent in the market. Butcher's were made with either half or full tangs with the fill tangs becoming more popular by the 1850's. On the later full tang knives 5 iron pins were used to attach the slabs to the tang. The three cutler rivets as still seen on these knives are post 1880.
Here's a link to a vintage 10" Green River with a five pin handle
http://img.auctiva.com/imgdata/4/2/5/1/6/9/webimg/605653845_tp.jpgWhile other knives such as Bowies, Spanish Belduques, daggers, etc. were available and used by some scouts and later period mountain men the most common knife, if commonality, is important to you then a 7-8" butcher style is the way to go. IMO the big curved skinners were common to buffalo hunters since in reality they are fairly specialized usage knife whereas the butcher was a more general purpose knife. While Russell still makes the Green River in both finished knives as well as blades only they need to be changed to five iron pin handles to be period correct and easy fix especially of you buy the blade only and do the handle yourself or cover the three rivets with a rawhide "repair". On the other hand 1870 and later vintage butchers by Wilson and Russell are often available on Ebay and other sources for decent prices with the bonus you are using a knife from the period you are portraying. A knife like that carried in a simple Indian style sheath like the one Charles Stobie is wearing would be a very appropriate and commonly used combo by white or mixed blood scouts for the 1860-1870's era: