At some point, windage looses from the huge fan in the 20kw head becomes more of a load then your electrical. I'd only go up 1 size from your engine
I've a 6/1 which pairs to a 3Kw, but I went with a 5kw, to be able to manage unbalanced loads a bit better, and not run the windings smoking hot. If I went to a 7.5kw it would still generate the 3kw my engine can produce, but now I'm running a bigger fan, moving more air, and maybe the engine can only pull 2.7kw
I'm just pulling number out of the sky here, but the alternator cooling fan is NOT a negligible load, I think 1 step up is enough unless you have a special need.