I think I'd run the risk and request 12" x 4" x 1.5"- 2" with the intention of refining sizes to suit my needs
Namely cut the resulting slabs to dimensions suited to razor hone, Knife hone and bench stones. I typically float between 8" - 10" x 2" - 3" x 0.5 - 1.25" depending on availability and end use (Predominantly chisel, plane iron and graver hones), with the best, cherry picked samples preserved for use as razor hone.
6x3 for hand honing which is what I do most. I do like lots of different sized for different uses though. The bigger bench are nice for Knife heavy work. Still hand hone on the finish though.