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.

Cheers,
-Arsheesh