I use Sigil, and when I put a book description into the metadata editor field, I format like so:
Code:
<p>First paragraph of description.</p><p>Second paragraph.</p><p>Third paragraph and so on.</p>
The paragraph code is all run together, exactly as shown! This renders paragraphs with one line between, much as you'd see in straight HTML with no styling. As for controlling font size, I don't know, as I've never tried to do that. I leave all that to the device UI.
I use a Sage and this is not giving me any extra space when I pull up book details. So if you want to try via Sigil, this may help. No guarantees, since I have no idea what, specifically, your desired look is.