Some maps already work pretty well with small user counts.
Maybe the smaller games should pick from a list of smaller to medium sized maps.
I think, the best way of choice would be to do more smaller pvp maps; back to the roots with asteroids, a couple of ruins and stuff, which work up to 8v8; and maybe also mapdesigns, which support both a bigger and a smaller playing field, by having different beacon placement or spawn regulations, would work too.
For larger maps, it would be nice to have 2 versions of the same map. 1 scaled down version for 6v6 and under, then the normal size for 7v7 and up. But could be hard to balance where beacons go, not sure