For those who care about details...

The calls for themeable stock items use GtkIconSize as a base, but then allow for arbitrary string-based sizes to be registered for new int values. Thus we can't simply extend their enum with +1 and use that directly. Instead we need to add in a lookup to translate at run-time from our extended enum to the types registered dynamically. This buys us quite a lot of functionality, and only needs extra work in a single file. First phase was to encapsulate the use of GtkIconSize in those two main locations. The second phase is to add the dynamic mapping of enum/int values. Then the third stage will be to change selected things (like the eye and the lock) to use Inkscape::ICON_SIZE_DECORATION.