https://senseis.xmp.net/?KorscheltsSize21Game
Here is a 21x21 game from the 19th century. It was either played by Japanese professionals or possibly composed by Honinbo Shuho. Apparently Oscar Korschelt, (who incidentally was probably the first Western Go player we’d call at least dan-strength today – he took a seven-stone handicap from the world’s strongest player, so make your own judgement) actually made a 21x21 goban and gave it to Shuho. Shuho came back to him with this kifu, which is 57 moves long.
If you want a little of my own insight into the game, moves 1 – 20 are an old-fashioned 5-4 joseki that was popular at the time; and the next ten moves also look perfectly normal for 19x19. To me it’s move 35, an early cap, that first shows at least Black to be valuing influence more highly than normal.