Generally, more civs makes the game more difficult. More civs means you are more likely to have a variety of AI personalities on the field, which increases the chance for wonder-hogging, missionary-spamming, runaway-ing... etc.
Personally, I consider standard size with 8 civs to be a "vanilla" or standard game of civ. It feels like there is just enough civs and just enough world space to expand, trade, engage in diplomacy... basically participate in all of the different facets the game has to offer.
It's really hard to define any "optimal" or "ideal" settings, because it really depends on your definition of the word. Does "ideal" mean most enjoyable game experience? Does it mean the earliest path to a certain VC? You get the idea.