The general thing about size is 25 gpm is nominally suited to 3/4" hose and fittings, 14 gpm is happy with 1/2" hose and fittings, 8 works ok with 3/8" etc., etc. That's the rough first cut - but it can all go out the window whenever it suits a manufacturer to do otherwise - like some tractors with serious pumps get outfitted with a cab and poof, the joystick and quick connectors have no place to live except under the step, so they use 3/8" plumbing. Oh, well, another good engineering theory trashed by market forces. Unless you use your tractor flat out all day every day, you will likely never really notice a size mismatch in the hydraulic plumbing. The exception might be if you run a big splitter off the third PB valve in a web.