Check out the FFXI Auction House database. Not only does it have listed the number of characters in each server, it will also give you an idea of what the economy is like.

I play on Ragnarok, which used to be the most populated server. Now that place goes to Odin, while it looks like Asura has the most healthy economy. It just goes to show how quickly things change in this game. But honestly, every server is doing pretty well, so I don't think it's really going to matter which one you choose. Like Miriel said, every server has it's fair share of helpful people, but you're also going to have yourself a healthy hit list of all the players that have wronged you in one way or another.