I'm no chainsaw expert, but most of the guys out here are using 14-20" saws, with most of them running 16 or 18" bars. The 20" is just too much saw to ride with for most, and the 12-14" saws take to long to nibble through the larger logs out here in the PacNW.
I have a Stihl MS180 with a 16" bar and its ok. The chain tends to dull really fast, its kind of driving me crazy. I always have a spare sharp chain ready to swap out and file for giving it a quick sharpen.
Recently I was reading somewhere the MS180 chain gets dull fast because the chain is so small, both in length and width. I forget what the size is, but if I was buying a saw for trail clearing now, I'd be looking at the next larger chain size, whatever saw that might be.
I'm also hearing good things about the battery powered saws, guys are carrying an extra battery and claim to be getting a lot of work done. I'm a bit skeptical on that claim, but its coming from guys that sound like they know what they're talking about.