Having good technique and a sharp chain is far more important than having lots of power. The biggest learning curve for me moving to a place where I cut my own fire wood was the saw maintenance.
I have three chains that I sharpen on the vise at home, and bring with me in case I hit dirt. Usually faster to change a chain than sharpen one in the field without a vise.
I have a Stihl 250 with an 18" bar. For trail clearing it's more than adequate, and honestly considering I lug it up and down mountains to do that, I would be happy with a 16" 170 for the purpose. I went for a hike and cleared a few dozen trees yesterday, none of which were larger than the 18" bar.
Back at home doing fire wood, the 250 is fine, though I do feel like I'd like something with more power at times when I'm ripping through a bunch of bucking. There's really nothing wrong with the 250 though.