The top of a standard king mattress is 76x80. Then decide the finished block size you are going to use. For example, if you use a finished block size of 12”, you would probably want to set the blocks for the center 7x7, resulting in a top (before borders) 84x84. The total border width would be 13”, plus. Here is how the plus might work. Add a 3” (finished) border all around. Add a 4” (finished) border all around. For the last border, add 6” (finished) on the sides you want to finish at 110 and 7.5 on the sides you want to finish at 113.
Me, I would make it 110x 110.