The first two
%_cover_width%
and
%_cover_height%
are available since Mp3tag v2.87b:
Since this part of the documentation, also this 3 are available:
%_cover_mimetype% --> Mimetype of first cover art in the tag of the file
%_cover_size% --> Size of first cover art in the tag of the file in bytes
%_cover_type% --> Cover type of first cover art in the tag of the file