Building my own splitter and need help with pulley size

Are you sure you can use a pulley with that pump? Some pumps don't have bearings that can take the sideways force a belt applies and are designed to be driven by a spider coupling.

If the mfg recommends a coupling, then the ratio would be 1:1 for the engine RPM. If you use pulleys, you might consider a pillow block and spider but that will really drive up the cost. I would pass on the pulleys and use a spider coupling.

In order to handle the torque of 5 hp. pulleys of greater than 4" diameter with double belts would be needed. If the engine is a newer horizontal shaft engine it will have predrilled mounting holes for a pump adaptor and lovejoy coupling.

I just scrapped a pully and belt system on my splitter. Definitely go with the direct connect using the lovejoy connections. You can find all the items you need at Northern hydraulics. My problems with the pulley system was constant loosening of the belts, Belts breaking, and the need to have a floating engine mount that could be moved out to tighten. I didn't have a tensioner on my set-up. I just rebuilt my splitter with a 16gpm haldex pump with the direct connect and it is like night and day. Vibration was so bad before with the pulley that after 2 hours of use I was tightening up stuff for the next 1/2 hour. Also with a pulley system the pump is next to the engine and spinning in the same direction as the engine. Most 2 stage splitter pumps are designed to spin on the shaft of the engine in essence going backwards...does that make sense? The connections ran me around $50 but your set-up is smaller so you can use the smaller mounting bracket and couplers. Pulleys just make another mechanical weakpoint when it is not necessary.