I'm not sure that I understand the question. You have a line drawn on your screen then in browse mode the line is the width of the screen. Click on the Position tab in the Inspector. Under Autosizing their are four locks. Try unselecting the right lock.
No, I've not drawn anything on the layout. I'm using the automatic Alternating Fill for the Body Part. Here's a link to a screenshot; I'd like the color bands to end where the red lines are drawn at the right edge of my print page.
You might try adding a traling grand summary part to your layout to fill in the unused space after the last record listed in the body.