I'm such an idiot for only giving half-an-answer
I had completely forgotten that in my PRSPlus css I have the body set like this:
Code:
body, body.calibre, body.calibre1, body.calibre2, body.calibre3 {
font-family: serif;
font-size: 0.85em !important}
This should catch most of the Calibre conversions. Thanks for making me think a bit harder, dwanthny and JSWolf.
In addition, during conversion, Calibre seems to replace epub css like
font-size:small or
font-size:10pt with a suitable
font-size:??
em; equivalent which is more likely to work the way you want it to with PRSPlus.