Very nice scan. The question of "how big should the scan be" is a vexing one, because it depends on the quality of the original. Scanning a low quality original at very high resolution doesn't get you any more detail, because it isn't there in the original; in fact, scanning at a too high resolution can create a file that looks worse.
What you've produced looks excellent. You can try scanning at a higher resolution to see the difference, but I suspect you're already at or near the sweet spot in quality with the scan you posted. No moire, no grain, looks very good.
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