One caveat with an electric helper pump is its flow rate... When you are running at higher RPMs some smaller electric pumps can't keep up and end up being a restriction. A fuel pressure gauge will show if this is happening.

-Jason

Just use a pair of vice grips, you're tossing it out anyway, right?
But to answer your question, 24mm and you'll need a stubby.

A dollar wrench cut down to fit in there for installation is fine. I think the size is 17mm. Usually just use vice grips on the old one to loosen it. Never use them to install the replacement. Someday I might get an old primer pump that is too tight to easily loosen with vice grips but not so far.

If you have some plumbing tools you might already have the wrench to remove it.
Members have suggested the use of a Basin Wrench for removal due the crampped space and the fact that the old style Hand Primer body has rounded corners on it that an Open End Wrench will often mush right over the corners.
However, do not use it to install as it squashes the Pump Body.
SDs have way less space to work in than the 123s.

vice grips worked fine, thanks

Ended up buying a cheapy adjustable wrench, cutting it off, and welding it to a busted socket to make myself a cheap adjustable version of one of those tools you can turn from a distance with a ratchet.

Worked pretty well.

That Basin Wrench is the best tool, beware the chepo ones tho, they can have such "fat" jaws they wont fit around the backside of the pump.