Intel iGPU die area and power consumption
It's not entirely a secret. They publish these photo's in their press kit:


(Intel Core i7-2600K source: pcmag)

And, since all parts of a family have the same die, you can read the specs to see the changes.
For example, if you compare these two chips: E3-1226V3 and E3-1230 v3.
You will see that graphics come at the expense of hyperthreading and still costs 4 watts more.

It's also completely irrelevant data if you're not developing motherboards. That's why they do not publish it to the end user.
Intel does publish register maps, for those wanting to program bare metal. But designing a board for these parts is such an effort that you have to ask Intel for design assistance.