Here are some graphics of ways I extend the width/length of 45" wide fabric when i'm doing backings.
I always allow enough fabric to match the top, then add at least 4-8" more (depends on how big it will finish) to that size to allow for "take up" during the quilting.
The inner gray lines represent the pieced top the pieced top. The outer lines are the edges of the pieced backing.
Jan in VA