Larger to Handle all
Agreeing with Gord here. Even with check valves(which restrict and fail) I would not do that. I went from 1/2" to 3/4" so that I could install a real seacock. Then with an elbow and a ''T'' valve I can supply both the engine and the A/C/heat. But never at the same time.
My head intake will also have a ''T'' valve to supply a washdown pump. Again just one at a time.