Isn't there an attribute to describe the breadth of <hr> tag ?

Answer 1

hr tag, changed it's definition from a simple horizontal rule in HTML4 to a more semantic thematic break in HTML5.
The old attributes are deprecated and you should style the hr with CSS.
See this comprehensive page with an example on w3schools:
https://www.w3schools.com/tags/tag_hr.asp

Answer 2

width?