If you were to follow the natural progression of the monk unarmed strike damage, it increases every four levels. Also, it seems to increase from d6 to d8 to d10 and then come back around to d6. Were you following that logic, this is what the damage dice would like all the way up to 41st level:
From there, you can look to the natural attack FAQ to determine how the damage dice increase when going from Medium to Huge size. In this case, 4d8 becomes 6d8 at Large size and 6d8 become 8d8 at Huge size.
Mind you, as Haladir mentioned, the game doesn't support anything above 20th level with official rules, so this is just an extrapolation based on what we have available. i.e. This is unofficial and just for fun!
I like figuring stuff out like this, so I hope it's helpful!