Personally, I find a 3" blade on its own often wanting in terms of length. Usually, that is for batoning and prepping a hearth for bowdril.
A four inch works great. I think, however, that 5" is the sweet spot. A long while back, when Bryan Breeden just started coming around these parts and introducing his pathfinder model he made the statement about 5" blades. "You can reach to the bottom of a peanut butter jar with a 5" blade, a 4 incher will get your knuckles full of peanut butter touching the rim". Genius! Yes, that is very, berry true!
Actually, contemplating this week, I think my Breeden Pathfinder, with its 5" but slender blade and 1/8" thickness is the most versatile knife in my collection. Big enough to do big boy things and small enough to be less noticable. Shucks, I can live with 4", but the simple facts are, if you measure a 5" blade and then you measure a 4" blade and you take the difference between the two, you have 1" remainder

Well them is the facts!