Every MUA I've used allows the reader to set a font size, so changing font sizes is 100% a feature of plain-text emails. Then they get the link the size they need to read it correctly and it's absolutely easy to read. This here comment is pain text. Is it hard to read this link:

http://microsoft.com/

I don't think so. I certainly didn't have to resort to HTML to make that link readable and clickable.