How to value market size? Where can I learn about market value of specific sector? What keywords should I be looking to being understand which reserch facilities publish relevant reports?

I'll put it simple: I'm building a startup, and I need to answer how much my market is worth. Where do I being this research? I believe this is a question every aspiring entrepreneur has to deal with, and I'm sure answers by knowledgeable people on this forum will become invaluable resource to the general community.

In my specific case, I'm looking at the b2d market where companies such as Twilio, Stripe, Github, Zencoder and StatHat operate.
You might want to check out Fermi estimation. XKCD did a very accessible example of Fermi estimation being used to estimate paint production. This method can be quite good at providing estimates within an order of magnitude of the correct answer.