It's going to be 6.4 inches. You measured the entire phone, not the screen so you are not taking the bezels into account.
Pixel 4 XL width 75.1 mm SCREEN width 68.5 mm. So bezels are 6.6 mm( 3.3 mm each side )
Pixel 5 XL width 74 mm SCREEN width 67.4 mm( assuming same 6.6 mm bezel )
Editing the pic to just the screen shows 19.7:9 ratio which means the SCREEN diagonal is about 2.41 X the width. Since the width is 67.4 mm 2.41 X 67.4=162.4 mm or 6.39 inches
And even using the size of the entire phone you'd get 6.7 inches. Length squared plus width squared = diagonal squared
6.05 X 6.05=36.6
2.91 X 2.91=8.48
36.6+8.48=45.08.
Square root of 45.08= 6.7 inches