If you do this and want to use the same code for every book, go to preferences in calibre, then choose common options and paste the code into the extra css box. Now, if you convert a new book, it will automatically put that code into the extra css box for you so when you click convert you can then click ok right away. You can also play with the font default size here too. I use 13 as my base font size with Verdana as my font in the extra css. I like the size of it on small.