I plumbed in a 0-5000 psi pressure gauge when I set my splitter (4" x 24" three point) up a few weeks ago because I was curious about what kind of force the wood was requiring to split. I burn only hardwoods; red oak, white oak, beech, and a little bit of hard maple, splitting both green and seasoned.
In order to monitor the pressure you need to watch the needle as it's splitting for it to spike, normally it will read zero because of the open spool design ofthe splitter's valve. With everything I've split I've never seen it go above 1200 PSI. The hydraulic system goes to 2700 PSI so there's plenty of reserve for what I'm doing.
If you're planning on a 4-way splitter (splits log into 4 pieces at once) then a larger cylinder might be preferred. My present setup only splits into two pieces though I'll eventually rig something up to try 4 way splitting.
