I depends on the shade of greeen. Light shades would work well with a small room. A bluish green provides a calming effect which is beneficial in a bedroom.
My bedroom is also relatively small and we have green painted on all four walls. I actually the paint. I wouldn't have picked it myself but, it doesn't make the room look smaller by no means and it is aloy better then having plain white walls with nothing on the walls.
It entirely depends on what shade of green you are looking into. If your room is small go for a lighter or lightest shade of green. A light colour will give a spacious look to your room.
In the right hughes, green is a great color for any size room. Just be sure that you don't paint the entire room green. Instead, use green as subtle hints and maybe paint only the bottom half of the wall green and the top a cream color.