I have a Harbor Freight Predator 6 HP on mine that runs just a hair above idle. Leave the drive belt loose enough to slip in the event of a snag. I have only had to stop and clear it once or twice in the 4 years I have used it processing about 4-1/2 cords/year. Mine travels about 18"/second. It has a 2-1/2" pulley on the engine to a 14" pulley on a jack shaft connecting to a 12 tooth sprocket driving a 60 tooth sprocket, which drives the conveyor sprockets and chains. My conveyour is 24' long and 18" wide with steel paddles. It was 36' long originally, but it was bent up prety good, so I shortened it. When I split it may get at most, 4 or 5 pieces of wood running up it at a time.I'll bet that 3 HP will be more than enough with a little steeper gearing than mine, so that it runs about half throttle.
Edit: Mine is geared low enough to start with the belt on. I did incorporate an idler pulley to tighten the belt and engage the power, but found I didn't need it, so a light spring keeps the belt tension just right.