I don’t think you will make much progress using a wood spliter to shear the wood against the grain . You would do much better with some type of saw. Maybe a table saw .
Before I made my chunker I would process wood fuel by using my chain saw, sawing 2-3 inch disc from a log and then us a hatchet to bust the disc into chunks.
As far as the size of the fuel I use everything from sawdust and spliters up to peaces the size of my fist . The average size that I shoot for is about the size of a cigarette pack.
Many pictures of my wood above at 1745