I play mostly Medium with some Large maps. Medium games for me is neither too short or too long and have a decent number of opponents. Large tends to take too long for my taste but I like having more special areas to explore.

I usually play on the largest size maps with  large numbers of ai opponents, but to speed the development of the ai I would suggest medium size as small map games usually end fairly quickly and do not get to develop all the techs.

harpo

I always play large maps with max AI.  I am using Ecomental Xlarge with 9 opponents.

It's pointless optimising until the resource generation bugs are worked out.  You'll be sitting there wondering how the AI sovereign is so godlike and then you'll look at their mana level - yeah wow, three hundred million?  No wonder they're so buff, they've been popping Destiny's Gift/Insight every turn, the cheating gits.

I want the biggest damn map possible.

XXL with ecomental - lush and ecology

I am wondering why you are optimizing for a specific size?

If you are doing automated game running, to see how the AI deals, I think I would do repeated runs with either random or continuously varying settings.

Optimizing suggests "over training".

Still, if you must, I would go for the game defaults.  (But, personally, I play with the huge map mod, and wish the map could get bigger than that, but I can already see my machine struggling...)

Edit: note also that I turn off the victory conditions which allow a single player to win without involving anyone else (spell of making and master quest), and these also reflect potential optimization issues (and so do all of the other options for map building).

I'm not sure what optimization is meant to be with the map size...as far as I know, the bigger map, the most optimization would requiere....

But after some large, extralarge and huge maps, now I'm playing medium maps with many AIs, to encourage land fighting and earlier battles...

Medium...it's just right for me...since if large map cost me many more hours and lag, slow game...

If you could add two new AI traits - Claustrophobic and Agoraphobic - then that might make things interesting when you have lots of opponents squished into a smaller map, or very few opponents on a big map.