The Kursi is the resting place of the feet, and the size of the Throne cannot be estimated

The Kursi, which Allah has attributed to Himself, is the resting place of His feet. This interpretation of the 'Kursi' that Ibn ‘Abbās (may Allah be pleased with him) mentioned is widely known among Ahl As-Sunnah, and it was authentically reported from him. Ibn ‘Abbās was also reported to have interpreted the 'Kursi' as knowledge, but this is not proven to be transmitted on his authority. Also what was reported on the authority of al-Hasan that the 'Kursi' is the Throne, is weak and inauthentic.
The Throne that Allah, the Almighty, is above is a great creation, and it is only Allah, the Almighty, who knows Its size and capacity.