<p>I'm doing some optimising for my site and am trying to figure out just how big some inline code can be before it can justify having it's own file instead of being written inline.</p>

<p>My guess on how to measure it would be that the amount of code (CSS or JS in this case) would need to be bigger than the HTTP packets sent and received to get a 304 response. </p>

<p>Please ignore the fact that it's a good practice to keep styles and javascript out of the HTML page and think only in terms of browser performance. :)</p>

Honestly, in terms of browser performance, separating your CSS & JS files out and properly setting expires headers so they are cached will
increase
performance, not degrade it. The browser will cache it and never request it again, which reduces the amount of data transferred during multiple page views.
Only if a majority of your audience is coming to your website with an empty cache
and
only viewing 1 page would inlining CSS & JS help.
See:
http://developer.yahoo.com/performance/
http://www.ryandoherty.net/2008/10/12/optimizing-openspacebook/