Large. Not too small, not too big.

I tried huge once in a campaign. It worked well on the campaign map, but in battle it was such a pain to navigate that quited the campaign.

I use a Radeon 9500 pro /9700, 128 Mb and have all optoions set to max detail, except vegetation where I don`t notice any difference whether if it is least detail or max detail in the heat of battle.