I suggest that the information on the website should make this clearer. I was wondering too–I’ve been thinking of a board 2in x 5in, which is less than 100 sqcm, but it wouldn’t be acceptable.
You could say: “Board must fit within a 10cm square” or “Longest dimension must not exceed 10cm” or something like that.
And are there restrictions on board shape? Cutouts etc?