in reality, how big of an area should I plan for my G scale Bachmann Christmas train ?
If you only intend to run a 4 foot diameter circle (measured to the centerline of the track) then you will need a 52 inch area to clear the ties.
I believe it comes with a circle of track. That circle is about 4.5 feet across. So an area about 4.5'x4.5' is the minimum. Add some straights and you will increase that dimension by the amount of track added.
Chuck