Database Size?

Depends on how fast your processor is and if you're willing to trade off having a huge amount of players for the game running slower.

For maximum players you should obviously have the largest database and as many leagues running as possible. At least have a nation or two on each continent.

I just go by the performance stars when choosing how many leagues, anything over three stars will be ok. Though I usually keep it at no lower than four but with your pc you should be able to load up a fair few. Plus you can add/remove leagues during the game.