I have a Stihl MS 290 Farm Boss with a 20" bar. I cut about 4 cords of firewood each year and it has performed well in both hot and cold weather.
I wouldn't go smaller than a 20" bar for any serious cutting. A 14" bar is great for limbing and trimming brush but IMO not up to serious cutting. While, in theory, you can cut through a log with the diameter up to twice the bar length by approaching from either side, it is a lot more work. When it comes to felling trees, having to work around the trunk to cut through could be downright dangerous.