1" by 4" by 8" is about as small as you should go. Mine is 1"X 4" X 14" and when they get warm I put in two 2 1/2 by 2 1/2 by 12" and I only do 5 at a time.
My plates are 1 inch thick. I have been pleased with the quench. It still amazes me how fast the plates draw the heat out.
I use the for cooling my liners for my slipjoints as well. When the piece has colled a bit I toss it over on the plates and I am able to handle it much sooner and keep working.