Hi RM33,
I have not checked which server has how many players on at a given time, this also will be infulenced by what time zone you run in.
I can tell you that Higher Ground POA in the Action section of Gamespy
highergroundnwn.net averages about 60-80 players at peak times which are normally 5:00pm US west coast or 8:00pm US east coast.
This will drop off to 30-50 players off peak thus EU zone when the US players are at work.
I believe our slowest zone at the moment is Asia with roughly 20-30 players on in that time zone.
Though I must say that sheer population size is not all you should look for, active updates, DM involvement, special events, friendliness of the players should all be considered as well.
If you do want to give HG a try, look for me Bin So Bad, I am on every night at 5:00pm west coast US and I would be happy to introduce you around and help you get the feel of the place.
All the Best,
Bad