For fairness, I assume that a whole tournament will have to be played at the same size. Varying between matches won’t give a fair comparison between multiple bots at all.
They could always do something like the best of three between each pair of bots, with each of the three matches being played at a different size - I think that would be quite interesting, and keep bot writers on their toes. They wouldn’t even have to release the set of sizes until submissions were in.
I do think that knowing a range would be helpful, though, and would also very much appreciate being able to change the board size when running the game engine.
There is also this quote:
I think there is a (very) over powered approach that could be used for this game, but which falls down if the size(s) for the tournament aren’t known in advance. (Hint: pretty sure, though not certain, that a NN can’t be trained for an arbitrary size board!) So that may be one of the things they are trying to prevent by not just having a fixed size or set of fixed sizes like last year.