Either that's a planet of enormous trees or very small humanoids. Although of course it could be about very small humanoids and larger trees than we're used to seeing. the world of sci-fi abounds with possibilities.

This book was followed by "Victory on Janus," in case you're wondering how it turned out.

Ah, here's some more information: it's the trees that are big. And the green dude is the hero,Naill Renfro, who has been turned into "Ayyar of Iftcan, a civilisation that had died more years ago than either Naill or Ayyar cared to think of."