The moon will be full for Christmas and it is a smaller one that you will see. This is the moon at apogee when it is farthest from the earth. Here are two images of the moon, the upper one at perigee when it is closest and the lower at apogee.

At apogee (Dec 27th) the moon is 50,000 km further from the earth than when it is at perigee which last happened on Oct 18th. The moon's orbit around the earth is eliptical varying from 356,400 km to 406,700 km from Earth.

Read more at Nasa