This really isn't a VB.Net question as the font size you want changed is in an HTML-formed e-mail message... That being said, you'd most likely use an inline CSS attribute:
Code:
"<p style="font-size:9px">Disclaimer: This is an official government communication. As the recipient, you are responsible for the lawful use of this information. This e-mail and any attachments are intended solely for the individual or agency to which they are addressed. </p>" + vbCrLf + _