I cut firewood (commercially) with a Stihl 660. That's 91.5cc and a 25" bar. For what I do I wouldn't get anything smaller.
I also have a Jonsered 2137 (40cc -16"). Originally for camp fire and 4WD recovery work (trees across the road etc). It cuts fine, but a bit slower then I need in order to earn a wage.
It depends on what you're cutting, but you can NEVER have too much power. Only too much weight! An 18" bar would be just right for home firewood work.
One thing I did find when I first bought a big saw. Get quality.
Got an Echo and within 2 hours it did a cylinder (shop fixed at their expense due Echo claiming not a good enough quality 2-stroke oil).
I can feel the vibrations more, it seems to weigh more and it cuts at half the speed of the Stihl, despite the the Stihl only having 11cc more.