Hello.
This page and Meditation throne state that "Barons or higher will need a grand meditation throne" (the nobility title is also mentioned in some other places in the articles).
It seems to be a mistake, as 'small' wooden throne can accomodate an emperor if the room meets the title requirements (6 columns, 2 braziers, 2 drapes, a piano, impressiveness 160+, fine floor), as seen on the screenshot below (the golden large statue is there only to raise room impressiveness).