UPDATED: info for the shot provided by Mr. Riedel
In keeping with the season, I thought it would be fun to discuss getting a large moon in the background. I saw an awesome photo by AP Staff Photographer Charlie Riedel (http://www.ap.org/kansas/charliebio.htm) exemplifying this:
To do this kind shot, you would need a telephoto lens, and the subject would have to be far away. The further away the subject, the bigger the moon in comparison to the subject (you will of course need a longer lens to see the distant subject). I contacted Mr. Riedel and he very kindly supplied the technical details for his amazing photo:
"The photo was shot with a 500 mm lens and a 2x tele-converter making it 1000 mm. It was shot at an aperture of f/8 at 250 sec at ISO 1000. The cropped photo was about 50% of the original frame. The moon size was exaggerated because it was shot from about 2 miles away compressing the spire against the moon."
That last line is important. Again - it isn't the long focal length by itself that leads to compression -- it's the relative distances between the camera, the subject, and the moon. See this related post on focal length: http://betterfamilyphotos.blogspot.com/2010/09/whats-point-of-having-different-lenses.html Here is a related post from Joe McNally: http://www.joemcnally.com/blog/2010/08/27/shooting-the-moon/
Another technique to get a larger moon (which may be used in conjunction with a long lens) is to crop the shot, although we would have a lower effective resolution. A perfect excuse, though for buying a Nikon D3x or a Canon 1Ds MkIII :)
What about the moon's position on the horizon? A moon that's low on the horizon actually isn't larger than one that's high in the sky, though it may seem that way. However, a moon that's low may look more picturesque with objects on the horizon beside it.
One more thing: to keep both the moon and the subject in focus, I suggest using a small aperture and focusing at the hyperfocal distance. Focusing at the hyperfocal distance will maximize the depth of field for the given aperture to keep objects at infinity (here, the moon) acceptably sharp. Using a small aperture lets us have a shorter hyperfocal distance. To find the hyperfocal distance, use a depth of field calculator such as this one: http://www.dofmaster.com/dofjs.html . If you're out in the field and you don't have access to the dof calculator, one way to do this is to switch to manual focus, focus to infinity, then move the focus closer gradually until the subject looks sharp. Make sure to check with the DOF preview button (otherwise the lens will focus with the widest aperture, which will have a farther hyperfocal distance). Note: It's probably going to be a challenge to do this because the viewfinder will darken considerably with a small aperture.