Change style: Font size and line

Hi,
I’d like to know if there is a possiblity to change the font size of the signature’s text to a smaller size. Tried some html-tags like <small></small> but this will be ignored and output just as it is.
Also, a solid line instead of — would be really great.

Would be very happy about any help with this!

https://wordpress.org/plugins/add-email-signature/