It depends on how narrow you make your wedge. If you design your splitter after a Tractor Supply splitter, you'll need at least 4" of inner diameter on your splitter. If you design it with a narrower wedge and a more efficient log stop design, 3" would suffice. Have a look at this website for ideas. They sell log splitters with even smaller cylinder diameters that split some pretty tough stuff! Remember, smaller cylinder= faster cycle time.What size cylinder will I need ton wise just personal use?
Most splitters use low efficiency/high power to function, wheres you might be smarter to chase the high efficiency/low power path to save on fuel and initial costs of the splitter.
On that note. Do not design your splitters hydraulic system with any sharp 90 degree turns. This will heat up your hydraulic oil faster, and it will slow down the cycle time of your machine. It will also increase the wear on your hydraulic components.