Its really a matter of preference.  Things to consider are ease of cleaning, the amount of space you have, and your weather.
  If you are going to have one off the ground, it should be small enough to clean without entering.  For instance, mine is eight by eight, with a large access door on each side, so with a hoe or rake I can easily reach the entire coop from outside.  However, mine is a wire bottom coop, so that is seldom a problem.  However, in colder weather, the chickens appreciate skirting around the bottom to avoid the drafts.  Some people like to incorporate the area under the coop into their run, and it gives them more run area without taking up more space in their yard.  
Coops on the ground should be tall enough that you can clean without having to get down and crawl, or have special features, like walls or roofs that open.  If you have cold weather and want to heat, a ground coop tends to be better to hold heat, also sometimes is cooler in summer if well shaded.