not sure of size, but as with most things in life, bigger is better.

we always had better luck with the round type of pump above on our ditch witchs. the wisconsin engines used a side mount diaphram pump that was a pia to get to when mounted on the trenchers. we just went to napa and got the elec pump and mounted on the frame upstream of the mech one.
we always got the 3-5 psi pump.

yeah my diesel has a lift pump on it which i thought of using but it's 7 to 9 lbs and my tractor i put one at the tank so when or if it ran out of gas i could prime it with out killing the battery first.