055 is measured to 178 m as per Google earth images. Then the pixel count is compared to 003's pixel count. And 307 meters at the waterline is, roughly, guesstimated.
Take into account that the shadow of the 003 clearly shows just how much overhang, over the waterline figure, there already is. That's why the measurement line doesn't go all the way to the end of the visible structure, but tries to extrapolate where the end of waterline bow would be, using the shadow.
On another note, it would appear the hangar module is not near the back of the ship but almost at the middle. Cetainly at the middle of the hangar length, where the hangar should be at its widest. So the 26-27m measurement of hangar width then doesn't bode well for the overall maximum hangar width, when the final product is done.
Waterline lengths for Forrestal and Kitty hawk are 300-302 m, while Nimitz's waterline figure is often cited at 317 m. So this 003 would seem to be somewhere in between, though a bit closer to Forrestal.
Of course, it doesn't say much about the overall size or displacement of the ship as hull shape might be different. Chinese designs do have a habit of being longer, compared to their width, than the US designs; likely stemming from otherwise inadequate propulsion to go for wide designs.
Using the same methods to size up and measure width of the ship, I am getting just under 39 meters at its widest. it's not quite waterline level as we know, so the waterline value might be less, perhaps 38 meters?