I think the size will depend on how many logs you want in each block and how many blocks you want to make.
Working from size down....
Let's assume your king size is approx 100x100. 10 inch blocks would mean 100 blocks (10 x 10), but then you have to account for seam allowance, so you will end up 97.5 x 97.5.
If your block is 10" square, a center block plus 3 sets of logs would mean 10"/7, not a pretty number.
Let's start again. Working from block up....
let's say we want 3 sets of logs in each block. We want each finished log to be 1.5" x 7 = 10.5" + .5" outside seam = 11" finished (but 10.5 when blocks are pieced together). So each log needs to be 2".
Just examples of how to work it.
If you want help figuring out what size your logs need ot be given other variables, just PM me. I love the math part of this.