So that is a question floating on LUGNET, and there have been some interesting responses.
I don't know the size, but I can give some thoughts -
There are approximately 3000 members of LUGNET presently, and that is a growing number.
There are approximately 70,000 downloads of BrickJournal for each issue.
BZPower has about 30,000 registered users.
Conventions draw from 100 - 400 registered attendees, while public attendance is 10 times that number.
There's a lot more information that is out there, but I'm just using these for examples.
What does this mean?
The community has a lot of potential for growth. There are many people who find this hobby interesting enough to look at and even pay for displays. The trick is finding people who are interested in building. And those people are out there.
There a LOT of AFOLs that still don't know about clubs and other builders. And there are the other avenues of finding builders - FIRST LEGO League is a good example. These are middle-schoolers who use MINDSTORM sets to take on challenges - the World Festival is an amazing event where people allover meet!
The bigger question is does the community want to grow, and how this would be managed.
My position is that yes, the community should grow. It needs to provide opportunities to let things grow, by providing information on clubs and events (including showing how to do events).
The community should be looking at other places to outreach also - there are other conventions, such as science-fiction conventions (that's where House of Bricks began) and local events - cultural festivals, for example. Much of this already happening, and this is good - and this lays down a ground work for the community to grow.
Do I see a convention that woudl have 1000 attendees? I do, but it would take around 5 years to reach that number. It would also require the help of the LEGO Group on a scale not seen before. Such a convention would be a longer event also, with some really neat possibilities for seminars and activities.
The danger presently is believing that 'we' are the community. The community is a lot bigger, but lurking.