You mean an <HR> line? I don't know that you can get it completely from side to side, but try adding WIDTH="100%" right after the HR inside the >. Lower percentages make it smaller, obviously. You can also add heights 1 through 6 to the same command. You can make it pretty big and wide.
The following was submitted by Dan Souza
If you're refering to a line of text, use the "justify" attribute with the <div> tag. This is only supported by version 4 or higher browsers
<div align="justify">Your Text here</div>
The text between the open and close <div> tags will stretch accross the page.