Originally Posted by DiapDealer
Your selected output profile in calibre has a lot to do with the base (default) font size of converted documents.
Also note that calibre has a font scaling feature that you can use for conversions. There's a wizard in the Look & Feel section of the conversion dialogue that allows you to "map" sizes from the input document to the output document.
Calibre user manual (font rescaling)
Thanks for your reply.
My trouble is about how to set the font-size by less than 1em (100%) in the body tag (or in .calibre as the above), which seems an important factor to decide the font size for other tags.