How to set the paragraphs that user composed in the email to align the same as the signature?

Hi all,
I would like to find out if there is a way to limit/restrict the length of the paragraph of what I've typed in accordance to the width of the signature I have created for my emails, using programming; be it XML or XHTML. This is so it will not over flows further as it looks untidy.

Hope you understand what I mean.

Thank you.