Look at the picture of the circles above. You can see that the little gray circle in the center has a radius of 1. The outside circle has a radius of 4. This picture extends the circle into a three dimensional sphere. Well, partly right.
Density also has something to do with it.
Review the page on Weight, Mass and Density . This page will help you to figure out the gravity that there is on the surface of your world. Mr Nordley has given us a formula that will allow us to come up with a gravity that is compared to earth's gravity. The density of the earth is 5.5 grams per cubic centimeter. Earth's density is unusually high for a planet in our solar system.