As I play Pariden a lot, I'm working with a magic-using race that takes a lot of time to get its spells and mana act together.  Long game's for Pariden.  I would think the length of the game is a strategic factor depending upon race.

Large methinks. Pretty please make a slider for monster aggresiveness The only thing that could ruin this game for me on the final few yards before release would be a tamer world. I assume the load save game crash bug and that stuff will be fixed as well

I play exclusively small or medium, so that's where my vote is.

I mostly play on Medium size maps, the smaller ones feel too crammed and on the larger maps the game just drags a tad to long for my taste

I would say optimize for the middle since it seems a lot of the other settings seem to be done that way.  I'm playing small right now just to get the hang of the changes but will be likely be playing more mediums later.

In any game like this, I always play on the largest map with the most players.  I've always enjoyed massive, sprawling games.

I will always play the largest size I can.

I almost always play a large map.   I prefer this since it usually leads to a longer game.

 Large maps for me. And I love long drawn out games.

I'm pretty much of the same mind as stein220, who I quoted above.  I have played most of my games on Small Maps, with an occasional game on a Medium Map.  One reason I play the Small Maps now, is to play as many games as I can, in the time available, so as to get the best feel for the game that I can get, in the short run.  Over the longer run, I expect that I will probably end up playing most of my games on Medium Size Maps.

Two Qualifications: I routinely play against one  LESS  opponent, than the default settings suggest, on both map sizes.  I believe this allows me to play a slightly easier game.  Also,  it helps (a little) to "improve" the feeling of the first two X'es (eXplore and eXpand) in this "quasi-4X" game that you have created.  Otherwise the first two X'es are under-represented.

Secondly, as I have a smaller, older, weaker PC at present, I think a Large Map would be too taxing (too slow) for my current configuration.  As far as that goes, I am not sure that I would ever really enjoy playing on a Large Map.  Like SciFi1950 suggests, the hard-core players, among the Beta Testers, may tend to play the largest possible maps; but the potenially larger, future audience for your game (of more casual players) may tend in the direction of Medium (or Small) Maps. 

So, Optimize for the Medium Maps. 

Small or medium.

The largest size maps should be optimized. I also think it would be best for you to play larger maps to figure out the memory issues with them so that we can all play the size map we want without the slow downs in the later game..