Plumbing... Need help

[sphelps]

Bigger diameter pretty much means more flow in almost any circumstance so bigger is usually always better.

1" sounds pretty good for that size pump, you should be able to find a 1" x 1/2" PVC bushing at most hardware stores, pretty common part but any plastic will work fine. Avoid brass as already stated as it's simply a copper and zinc alloy; copper is poisonous to coral.