Thanks. The font size key did the trick for the font size but now I get the line height changing all the time. It's getting set to 1.2 without any unit named. I expect it means 1.2em.
I'm trying to make AZW3/older mobi packages, that is, I'm using the option to make one file that has both formats (I have older and newer Kindles in the family).
If I make files with only AZW3, then I can edit them afterwards. But with the combo files, editting them removes the older mobi format so I can't edit them afterwards.
I going to try Kindlegen and see what I get.