Our pal Der8Auer has gotten his hands on one of the big Skylake X parts that are launching later this month and done the needful with a delid to show off the die:
This is the die that is used for the 12 - 18 core end of the Skylake X (and Xeon W) lineup that Intel labels as an "Extreme Core Count" (XCC) die. The only bigger Skylake silicon is for the Xeon Gold/Platinum lineup that goes up to 28 cores.
Given that the package has a long dimension (vertical in that photo) of 52.5 mm and a short dimension of 45 mm, I have done my usual number crunching to come up with a die size estimation of about 491 mm^2
(roughly 21.77mm high and 22.56 mm wide).
I have already estimated the die size of the "small" Skylake X parts at about 327 mm^2 here for comparison
So, given that the "small" Skylake X scales up to 10 cores the "big" Skylake X with 18 cores occupies a die that is almost exactly 50% larger at 491 mm^2 / 327 mm^2. That's a 50% area scaling for an 80% core count scaling because the uncore area occupies a fixed amount of space across both processors so the larger chips can add additional cores while the total die area increases at a less-than-linear rate.
We'll see how it performs later this month. Early indications are that it is a strong overclocker.