These three horizontal rules are defined in pixel widths.

50 pixels:
75 pixels:
100 pixels:

And these three are using percentages:

50 percent:
75 percent:
100 percent:

Let's see how these look when we use the align attribute

50 pixels, aligned left:
75 pixels, aligned center:
100 pixels, aligned right:

Here are three more using the size attribute:

size="10":
size="20":
size="30":

Here they are again with the noshade attribute added.

Just a plain horizontal rule with noshade added: size="10":
size="20":
size="30":