The area of your game board is = 126 * 90 = 11340 cm^2;
A small suares has x^2 his area; where x measure his length;
Prime factorization
11340 | 2
5670 | 2
2835 | 5
567 | 3
189 | 3
63 | 3
21 | 3
7 | 7
1
11340 = 2^2 * 3^4 * 5 * 7 = 2^2 * ( 3^2 ) ^ 2 * 5 * 7 = 18^2 * 5 * 7
We observ that the possible length of the side is 18( we have 35 small squares ).