1. Grab a build at
http://commondatastorage.googleapis.com/chromium-browser-continuous/index.html2. Locate devtools_frontend.zip file located beside the build archive, extract it into a clean folder
3. Edit the styles (inspector.css or similar).
4. Start Chrome using --debug-devtools-frontend=<full path to extracted folder>
Happy hacking! Note that there are known issues with using devtools_frontend such as Heap Profiling and Pretty Print feature won't work. We are working on addressing those. Btw, if you come up with a convenient way of toggling the font size, please let us know and/or contribute your change into WebKit / Chromium. That way it gets into Chrome and you won't need to hack it again!
Regards
Pavel