Unless you have some seriously tough wood 5" cyl on a skidsteer is overkill. Most skidsteers have fairly high pressures, in the 2500 to 3000 psi or higher range. That kind ofpressure will get you some serious tonagewith a smaller piston.
I have a 4" ram and its relief is at 2500 psi and nothing around here has stopped it yet. Think its about 18 ton.
I also run it on a 24 gpm high flow loader and still find that 2000 rpm is needed to get decent speed. A 5" on a std flow loader will result is slower cycle times, which will have you running you the engine wot trying to get some speed out of it, imo
You need to figure your cycle times with the pump you have and the cylinder you want. Then decide if you can live with the cycle time to get the tonnage you want/need.
If you design for 30 tons, but only need it once in 100 blocks, remember that the other 99 times your are going to have to wait for it to make the stroke.
Also plan for a 4 way wedge, your going to want it. If I was building one to run from in the cab (upside down model) I would also make it so I could flip it up right (mount to the qa plate with a 2" reciever, then you can use the qa plate to spot trailers to) and plan to put a valve on it too. I think with the upside down model you will need someone on the ground constantly clearing the split wood away and lining you up more, but that just my opinion.
Ken