My default font for email text is 12pt. I find that if I change the quoted text to 10pt, the distinction between quoted text and response text is markedly better improving the readability significantly.
In addition, sometimes the quoted text lines overflow the width of the response leading to vary scrappy layout, which can be fixed with Ctrl_J, although this tends to lump all following paragraphs into one large unreadable paragraph. By reducing the text font, there is no need to reformat and this problem is completely avoided.
My suggestion is therefore to provide a parameter that allows the font size of quoted to be specified.