The other way to put that measuring line on the screen (as in the previous post by Jeff) is to touch the screen with two fingers at (almost) the same time, at the approximate endpoints of the line you want drawn. The first finger to touch will be the anchor (red dot), and the second finger will be the target. Hold for ~a half second or so for the line to appear.
As Jeff said, once the line is on the screen, you can grab the endpoints and move them around as required.