Sakamoto Ryoma is also known as Saito Hajime. According to Yakuza Fandom, he is the protagonist of Ryu ga Gotoku Ishin! and its remake, Like a Dragon: Ishin!
The same fandom mentions Sakamoto Ryoma as a 31 year old Japanese warrior. This information can be seen under his character “biographical” information section.
Yakuza Fandom does not give us an exact height for Sakamoto Ryoma, however, the fandom mentions that Sakamoto Ryoma shares an in-game appearance with Kazuma Kiryu.
Kazuma Kiryu is the former primary protagonist of the Like a Dragon (formerly Yakuza) series. He has a complete physical appearance description available on his character profile in Yakuza Fandom.
The fandom mentions him as a Japanese man with a height of 184cm or 6 feet.
Since Sakamoto Ryoma and Kazuma Kiryu shares the same appearance, we assume that Sakamoto Ryoma is also 6 feet tall, both in the original and the new remake.