There are TIME and TSIZ fields in the ID3 spec but Mp3tag writes to TXXX fields instead.
These fields are not listed in the help page list of supported fields
TIME
SIZE
ITUNESMEDIATYPE
TVEPISODE
TVEPISODEID
TVNETWORK
TVSEASON
These fields are out of alphabetical order in the help list
ENCODINGTIME
ENCODERSETTINGS
ISRC
WWW
WWWPUBLISHER
WWWPAYMENT