Where should I look for market size information on Social Media/UGC/ Web 2.0 or do you have any?

If you are looking for sites that can help you with social media marketing then check out the likes of:
www.viperchill.com
www.pronetadvertising.com
www.sphinn.com

If you are just looking for traffic and member figures check out the likes of:
www.alexa.com
www.comscore.com
www.compete.com
www.techcrunch.com

Hope that helps

Cheers,
Glen