I cut up most blocks that are from a log to about 2.5X1.5X6". After fully drying, I trim all the sides to get clean flat surfaces and then have it stabilized. Once back from the stabilizer, I trim a tenth inch off each side to see the pattern well, and assure they are straight and flat. They end up various sizes around 2.25X1.25X5.5. On some I cut the blocks in half to get two 1.25X1X5.5 smaller blocks. These are good for small knives and kitchen knives.
On some other blocks, I cut them to 1.5X1.25X5". This is usually wood that is already dried lumber and just needs stabilizing. After stabilizing, I just give them a quick sanding.
On block for scales, I go 1.5X1X5". They get cut in half after stabilizing and cleaning up on the sander.
When a large piece of wood is really filled with a bold pattern or burl, I cut large blocks and have them stabilized whole. Often they are 12X3X6" These get cut up into exact size blocks as I need them. By carefully cutting the pieces, I can get more handles as well as small handles and bolster blocks from the cut-offs.
One thing to always keep in mind is - Don't be afraid of cutting away and discarding wood before sending t off. If you need a handle to be a certain size, cut the block just about 1/4" larger on all dimensions before stabilizing. It is foolish to stabilize large blocks that will end up being cut into smaller blocks and sawdust/scraps when you are paying $8-10 a pound for the stabilizing.