It's been many years (decades) since I've used a horizontal rule but I don't remember that there were any problems with defining the width, either setting it to run the width of the page or a desired number of inches. I wanted to use one in an article I'm working on and simply inserting <hr /> between paragraphs gives me a short line, not much more than an inch by eyeball measurement. I want it longer. In the admin side Editor window it spans the width of the page but not on the public side. My fall back would be to make a graphic image instead of using <hr /> but now I'm curious because I don't remember how to do it.
Thank you, yet again.