If you want to use the center as cut size 8-1/4, and use a 5 x 5 grid, then the cut size of the pieces should be:
1A: 3.08 x 3.08
1B: 3.08 x 8.25
1C: 8.25 x 8.25
The block would finish at approximately 12.91x 12.91 inches - 13.41 x 13.41 unfinished
These are not "impossible" numbers to work with - just a bit unconventional for most people used to working with 1/8.
I would suggest either sashing the coffee pieces to make them a bit bigger, or cut them down.