You can send letters to Song Kang-Ho with this address:

Song Kang-Ho (Prisoner No. 219)
Jeju Post Office PO Box 161, Jeju City, Jeju Peace Island, Korea 63166

There are no specific regulations about letter size, but unfortunately, sending books is not allowed anymore.

Song Kang Ho and Ryu Bok Hee visited the remaining part of Gureombi Rock (inside Jeju Navy Base) on March 7, 2020, the 8th anniversary since the blasting of Gureombi Rock in 2012. On March 30, 4,700 people signed the petition calling for the two to not be imprisoned during their trial. Ryu Bok Hee was briefly detained and released on the 30th. Once again on April 3, when the court reviewed the legality of Dr. Song’s arrest warrant, 2,448 people signed the petition calling for his release. The text of those petitions is here: http://savejejunow.org/petition-supporting-song-kang-ho-and-ryu-bok-hee/