I am guilty of using "midsize" and "full-size" interchangeably with modern sedans, but if we all faced facts and ignored emotions, the North American full-size segment from the 1950s to the '70s (and a select few body-on-frames and granny cars after that era) were the TRUE full-size segment, and the only real full-size sedans made today are the long-wheelbase versions of the luxury flagship sedans.
The G80, however, is different from its supposed "competitors" in the so-called E-segment. It's essentially a "short" (at 118.5 inches of)-wheelbase G90 with a notchback design. It rivals a standard-wheelbase A8, 7 Series or S-Class; it's not a competitor for the A6, 5 Series and E-Class as Genesis fans convince you to think.
These days, even F-segment is filled mostly with 'BIG' midsize cars (but barely any true full-size cars). The Cadillac XTS and Lincoln Continental were just that, however the CT6 and MKS (as well as their DTS, DeVille, Fleetwood, and Town Car precursors) were for sure full-size cars.
(57.7x75.0x196.7)÷100 = 8512 sq.in. (Genesis G80)
How does that compare to an A6, 5er, E-Class, ES, or Ghibli? (I know about TLX and CT5, but those technically compete in D-segment, and I'm bringing up only the more popular cars on here.) I'll add the XF as well.
(57.4x74.3x194.4)÷100 = 8291 sq.in. (Audi A6)
(58.2x73.5x195.8)÷100 = 8376 sq.in. (BMW 5 Series)
(57.8x73.7x194.3)÷100 = 8277 sq.in. (Mercedes-Benz E-Class)
(56.9x73.4x195.9)÷100 = 8182 sq.in. (Lexus ES)
(57.5x76.6x195.6)÷100 = 8615 sq.in. (Maserati Ghibli)
(57.3x74.4x195.4)÷100 = 8330 sq.in. (Jaguar XF) - NOTE: width excludes outer edges which combine figure to 78.0"
(56.9x74.6x195.6)÷100 = 8303 sq.in. (Volvo S90 EUDM)
As we can we above, inside the supposed E-segment it most ideally competes with Maserati Ghibli (which actually is an F-segment car as well). Also, Chrysler's labeling of the 300 and its Dodge Charger sibling as "E-segment" is incorrect and false advertising, they're actually F-segment twins (but like the G80 and Ghibli, on the shorter-wheelbase side of A8/7er/S-Class).
Now, on to the F-segment market sedans (bar LWB versions):
(58.5x76.6x203.6)÷100 = 9124 sq.in. (Audi A8)
(57.8x74.9x200.7)÷100 = 8689 sq.in. (BMW 7 Series)
(59.2x76.9x203.9)÷100 = 9283 sq.in. (Mercedes-Benz S-Class)
(57.9x74.8x206.1)÷100 = 8926 sq.in. (Lexus LS)
(58.3x76.7x207.2)÷100 = 9265 sq.in. (Maserati Quattroporte)
(58.5x75.0x198.6)÷100 = 8714 sq.in. (Chrysler 300)
(57.1x74.6x200.4)÷100 = 8536 sq.in. (Volvo S90 USDM)
We can rest assure that the G80 is an F-segment car. However, there is no true E-segment offering in Genesis' lineup, so it technically does fill that void too. It's only barely smaller than the current generation 7 Series, which is smaller than the 300.
The Maserati Ghibli is definitely more of a 7 Series rival than a 5 Series rival.
OUTSIDE of the LWB section of F-segment, we can assure that the Audi A8, Mercedes-Benz S-Class, and and Maserati Quattroporte are the only ones close to being full-size. The current-gen 7 Series and 300 both are upper-midsize cars, but let's hope that the next-gen models are true full-size beings.
Interestingly, the "full-size" S90 USDM model is smaller than the "midsize" Ghibli.