HTML Attribute for <HR ...>
SIZE ="height expression"

Usage Recommendation
Use it, but don't rely on it.

SIZE indicates the height of the rule. (I guess calling it "HEIGHT" would have just been too easy.) For horizontal width see WIDTH. Compare some of these sizes: 

<HR>
<HR SIZE=1>
<HR SIZE=5>
<HR SIZE=20>

which gives us

Browsers will generally not render an <HR ...> any bigger than 100.

<HR WIDTH="..."> >