MobiPocket does not internally support font-size, but MobiPocket Creator is probably converting it to something that MOBI handles. What MOBI has natively is the big and small tags, from
Kindle Formatting: The Complete Guide by Joshua Tallent:
Code:
<p><small><i>The Four Million</i>, copyright © 1906 by O. Henry.</small></p>
These tags can be nested, with each providing one jump in font size.
You could also try your original version but run through Calibre to convert it to MOBI. Calibre does a better job than MobiPocket Creator on converting HTML tags and styles to MOBI.
Since the Hanlin only has 3 font sizes for MOBI, I'm not sure how these are mapped. I'm pretty sure you would need to be on the middle or large size for a smaller font request to have any effect. Most other devices using the Java MobiPocket Reader have many more font sizes than this.