[Peter De Smidt]

The best way to tell is to get the kids to sit or lay as intended in front of a wall. They should be the same distance that they'll be from the background. Now focus and frame as you will for the real photo. Tell the kids to go play, and make sure not to change anything. Have a friendly helper put a piece of masking tape on the back wall, just when the tape can be seen through the view finder. Measure the distance. Add about 1 foot and you'll have the size needed for your background.

Personally, if I'm going to the work of making a background, I make it 9ft wide, as that's a standard size and it fit's in my room, but that is a pretty big thing to keep around. If you paint it you have to carefully roll it up. A dyed muslin can simply be stuffed in a plastic bag.