Scale has always been an issue for me as well. A good rule of thumb is to keep a map of the earth on hand and use it as a reference. By familiarizing yourself with the geography of an actual planet it will help you become more proficient at creating whole new planets. Also, is this a Pangea like planet? Does that single land mass represent several separate plates? If so, you should have mountain ranges in areas where the various plates bump up against each other.