The total market value is the most objective indicator of size.
Bitcoin - 10,936,775
Litecoin - 112,605
Namecoin - 11,298
PPCoin - 3,646
Novacoin - 2,706
Terracoin - 2,478
Units in BTC. From dustcoin.com
Not in my opinion. Total market value is too unreliable. I would say how many people are mining it would be a better indicator, and if you look at coinotron, it tells (a biased towards pool miners) but a different story.
If you have 1 million coins and only one person is willing to sell 1 coin and they sell it (or ask to sell it) for $100 then you have a market share of $100 million dollars. It's too variable an indicator to be objective.