I did program an 8x3 for BGA. It's selectable when issuing a challenge. There was some discussion about recommended sizes early on. One consideration is that game length grows exponentially with board length. With too short of a board, first (or second) turn advantage could become an issue. But certainly any even length board with any height stacks would be playable. The stacks don't even have to have uniform height. They could be taller in the middle for example.