While the font-size cannot be changed for text directly inside pre tags, you can always wrap that text in another element (a span, for example) and change the font size of that element.
For example (this is inline-styles, but could be put in CSS if you wanted):
<pre><span class="inner-pre" style="font-size: 11px">
Blah blah
Multiple lines and no br's!
Oh yeah!
</span></pre>