I find creating planets that are unique to be a very important part of being a good GM. These give options for unique factors if you are stuck. Please give comments with suggested additions. Also, what I consider to be the difference between worlds and colonies are that colonies have a small enough population that they extend across a part of the planet, while worlds extend throughout the entire hospitable area.
Step 1: Population options
Small colony-1 hundred to 13 thousand (1-3 small cities and maybe a large one)
Medium colony-13 thousand to 70 thousand (4-20 small cities with a few large ones, too)
Large colony-70 thousand to 2 million (21-1 thousand small cities with 5-1 hundred large cities)
Small world-2 million to 1 billion
Medium world-1 billion to 20 billion
Large world-20 billion to 100 billion
(While many planets are larger than this, their existence should be well known, so creating one of this sort would be illogical)