So I beat 11 computer players on a huge map FFA, marathon, level 6.

Would a level 7 small map FFA be easier or harder?

I'd say harder. If you mean large or standard. Probably even or easier for small or tiny maps.

It's hard to tell on marathon. Usually the AI tends to be really weak early on, because of how long it takes to rebuild the military units that they lose, but if a Civ manages to become a runaway and get enough land to swarm you, they tend to become ridiculous later on. On smaller maps, that doesn't really happen - in fact, on small marathon maps it's quite possible to just wipe out (or cripple) the whole world before entering the Renaissance-era, even on Deity.

So overall it depends very much on your playstyle. If you're aggressive, then it'll probably be easier, if you plan on playing passively, then you'll most likely have a harder time.

I find bigger maps are usually a lot harder, Deity on standard size I can win about 50% of the time, Immortal on Huge I can win a fair bit less than that. The AI bonuses allow them to expand faster and further so huge maps can often be a big handicap towards human players.

Try the Immortal small map, worst case scenario is you lose, you'll learn from it and become better as a result

I can confidently win on Emperor with any VC of my choosing with any civ. Yesterday I tried Immortal and got caned twice early on. The jump from Emperor to Immortal is quite big imo and you hope you haven't just inadvertently forward settled a warmonger....

Mistakes you can get away with on Emperor you can't on Immortal. Science is key so get those universities built asap and the specialist filled. Once you are the science leader(bpt) you become a RA target so many DOFs. If it seems your neighbor will invade bribe his neighbor to DOW him.

Great! Now your next step is to put those eleven AIs in a single team against you on a huge map!