You don't need to clear an area, you need to build an area...
I recommend world painter for my RPG worlds (coz I love enormous mountain ranges)
MC edit is pretty good for editing, but not generating. Use it for building transporting,
Long buildings and railways, mapmaking and filter effects.
For just building, we have flatlands and plains lands.
For actually clearing trees get treecapitator, use mcedit filters .
Don't rely on fire, you don't control that.
Also, for realistic builds, turn on survival with an iron or diamond axe and just play real till you get bored.