If my mathematical calculations are correct, a finished 12" block would be double the size of a finished 6" block, so each of the two strips would need to be 2 1/2" wide, with the width of the sewn two-strip set being 4 1/2" wide. The total length would be nearly 30". If I have miscalculated, please let me know.
For a finished 10" block, I am playing around with strips just under 2" each. I'm still playing with this and tend to overthink the process. I'm giving up tonight and will look at it again tomorrow or the next day, depending upon my schedule.