I also use the brute force method fogice mentioned. Then I get out my big sledge hammer and fix other things (things that calibre's books converter doesn't handle) with the following regexp for the css file. I do this in Sigil. The following goes in the upper search and replace box and in the replace with box is nothing. So all of those css directives are removed, regardless of the value. Afterwords all text is the same size, except for h2, h3, etc. (although many authors format headers with the p tag so they end up not looking as nice). One of the main things I'm after is being able to open a book and not needing to fiddle with the font size slider. The page-break stuff was recently added after I had a book with weird page breaks.
Code:
( display: block;
| font-size: .*
| letter-spacing:.*
| line-break: strict;
| line-height: .*
| page-break-after:.*
| page-break-before:.*
| page-break-inside:.*
| vertical-align: .*
| white-space: pre-wrap;
)