[Sirtel]

You can only get the same font size by conversion if you're willing to get rid of every kind of specified font size in the book - that also goes for font size in chapter headings, blockquotes, footnotes and so on.

If you only want an unified size for main text and leave other cases alone, then that can't be done with conversion. You'll need to edit the css/html manually for every book.

All this also applies to line height.