I think it would make more sense to be able to return the default values of all user preferences, not just thumbsize.

Although to make sense of the value for thumbsize (either the default or the value from a particular user's preferences) you also need to know the configuration of $wgThumbLimits and $wgImageLimits.

Or all sitewide config variables that can also be obtained by external inspection of a public MediaWiki instance. ;-) But yeah, I agree that a less piecemeal approach is preferable.

Change 100409 had a related patch set uploaded by Anomie:
API: Add image sizes and default user prefs to meta=siteinfo

https://gerrit.wikimedia.org/r/100409

Patched and will go out in wmf7. Yay. :-)

Change 100409 merged by jenkins-bot:
API: Add image sizes and default user prefs to meta=siteinfo

https://gerrit.wikimedia.org/r/100409