There are three variants of Skylake-X (SKL-X) and Cascade Lake-X (CSL-X). While CSL-X chips do have some minor changes to the underlying design (like hardware mitigations of some side-channel attacks), fundamentally not much has changed. So the following should apply to all chips of each family.
The LCC (Low Core Count) variety of Skylake-X has up to 10 cores. This is the die used in Core i9-7900X, Core i9-9900X, and Core i9-10900X. It's also used in the Core i7-7800X, Core i7-7820X, Core i7-9800X, and Core i9-9820X.
HCC (High Core Count) Skylake-X CPUs have anywhere from 12 to 18 cores. These chips (not counting Xeon models) consist of the Core i9-7920X, Core i9-7940X, Core i9-7960X, Core i9-7980XE, Core i9-9920X, Core i9-9940X, Core i9-9960X, and Core i9-9980XE from the Skylake-X family, and the Core i9-10920X, Core i9-10940X, and Core i9-10980XE from the Cascade Lake-X family.
Finally, the XCC (Extreme Core Count) models have 20 to 28 cores. These are used exclusively in Xeon chips -- they're not intended for enthusiasts. Of course there is the Xeon W-3175X "enthusiast" chip that has the full 28 cores, but at $3,000 I'm pretty sure no one bought it for gaming purposes.
But the question I had is: what are the actual die sizes of these various chips? So I did some sleuthing and came up with the following.
GamersNexus has some nice shots of a delidded Core i9-7900X next to a ruler. The measurements show a size of approximately 24.5 x 14 mm, giving a final die size of 343mm square. That's the easy one and you can find quite a few other places with pictures and estimates of die size. It's probably accurate to within 1-2%, which is fine since transistor counts are probably estimated in a similar fashion.
Skylake-X LCC Die Size: ~343 mm square
Overclockers where user batboy delidded his Core i9-7920X, and der8auer also delidded an i9-7920X. We do know the package size for all the Skylake-X chips (it's officially 52.5 x 45 mm, which is accurate to within ~0.25mm I assume). Using that and the delidded images, I estimated die size.
The der8auer image is better for this, and gives a package width of 684 pixels and a die width of 344 pixels, with a package height of 787 pixels and a chip height of 329 pixels. Using simple math, that works out to a die size of about 22.6 x 21.9 mm, or 495 mm square (plus or minus 5 mm square).
Skylake-X HCC Die Size: ~495 mm square
The image shows a width of 763 pixels for the package and 216 for the die width, with a package height of 538 pixels and a die height of 302 pixels. Crunching the numbers, that gives a final die size of approximately 21.7 x 31.7 mm.
Skylake-X XCC Die Size: ~688 mm square
Obviously there's still a bit of wiggle room, but the above is about as good of an estimate as I can provide. Equally obvious: Skylake-X is a big CPU in its LCC incarnation, and even bigger at HCC. Around 500mm square in size is one of the larger CPUs Intel has made. And of course, it can't hold a candle to the XCC die size of nearly 700mm square.
My real question: Just how big will Intel's Xe Graphics HPC variant be? That wafer shot from Raja, along with some of the things IntelGraphics has tweeted, suggests it will be the biggest chip Intel has ever made. And yes, it's going to be very expensive -- even more than Xeon XCC CPUs I'd wager.