Fix issue where "{Fnnn, size}" renders as though "size=full"

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