Summary: We interpret "size" as "size => true", and "true == 'full'", so we hit the wrong branch in the switch(). String cast explicitly.
Test Plan: Typed `{Fnnn, size}`; saw it render as a thumb instead of full.
Reviewers: btrahan, codeblock
Reviewed By: codeblock
CC: frozendevil, aran
Differential Revision: https://secure.phabricator.com/D4323