Welcome!
If one were to assume a linear progression, one would assume 250mm2 250mm2 split with cpu/gpu.
If one believes as I do that future consoles will rely on the GPGPU for CPU assistance, then part of the CPU die budget would go toward the GPU.
How aggressive this shift is is anyone's guess, But I'd assume a smart route would be to take an existing off the shelf gpu, and mate it with a custom CPU solution scaled from existing Cell/Xenon tech.
This will enable the relatively large GPU to be binned from a desktop chip which perhaps couldn't cut it at 925MHz, or perhaps doesn't have all of the 2048 ALU's working.
Thanks for the welcome. Your reasoning from a typical progression makes sense. However, from what you're predicting, while sensible, doesn't mean anything...
In fact, I don't think you can go by the past at all anymore. It's a different console market these days, and I firmly believe that Microsoft will go for a small and quiet, less powerful console, which, whenever Sony releases there's, will follow suit.
That's the main reason I was saying why is that the ballpark, simply because there's a good chance that the end result is smaller and much less powerful. I think you, and many others here, are way overshooting next-gen specs, in my opinion, almost in the same way that the Wii U thread is severely underestimating the specs.