Please help answer these few photography questions

1: The sides will be the same when cut in half, but the closest number that could be guessed is 3in

2: Wont matter much, they’re both the same

3: My speculation is that the Black square in 4x6. In order to know the size of a white square we have to do the following

4 + 4 = 8 left vertical side
6 + 6 = 12 upper horizontal side

So we have an The white square is 8x12