The Box piece was a happy accident, I think I spent about 5-6 hours on it. The Hunt took about 10-15 hours I think, it is hard to give an estimate on it because I already had painted most of the background previously.
Well my dimensions is not really anything crazy. It is normally about W:3500 Pixels - H:1500 Pixels and then when I am done painting I downscale it to give it a crisp look and sharpen it.
All it comes down to is just how many hours you spend drawing. The more time you spend the better you will become. I can't remember if I already gave this link to you? In case I haven't here is some great tutorials!