This depends on how you enjoy your game. Personally I like really large maps but with only a few things in and with large voids between stars, so that space actually feels like space. So my favouritge set up is insane map size, tight clusters. Stars, planets, resources set to uncommon or rare. Pirates and extreme planets set to common to spice things up.

I choose only a handful of opponents too, maybe five majors.

But be warned, this is a set up for a longer game.

I have the same preference. Big map but very sparse. Makes everything more special and valuable and worth fighting over instead of the "yet another, ho-hum" experience.

The problem with the "tight clusters" setup is that it's not very good in generating balanced starts for the factions whereas the "scattered" setup usually does a better job in giving everyone a reasonably equal start but then you lose the "space is big and empty" effect.

To solve that problem I modded the MapSizeDefs.xml. The tag "Base" in each map size directly gives the number of star-level objects on a map. If you decrease the number you get sparser maps and with the scattered setup everything tends to be further away from everything else. My mod was to divide the Base value by 5 for every map size which "stretches" the space quite a lot, but that's just how I like it. Effectively you are playing a smaller map but where the map is "zoomed" so that each hex represents a smaller distance (and thus making ships slower btw).