How big is a 65-inch TV? Dimensions and the step up from 55 inches

The diagonal, 165.1 cm, is measured across the picture only. Every inch of diagonal adds width and height in the same 16:9 proportion, so the screen grows in both directions. Going from 55 to 65 widens the picture by 18%, and the height grows by the same share.

At 4K the panel gives 68 PPI. From the 40 degree seat of 2.0 m (6.5 ft)6.5 ft (2.0 m), the individual pixels already blend together.

Choose 65 inches when the sofa sits about 2.7 m (8.8 ft)8.8 ft (2.7 m) back and the room can take the extra width. The panel measures 143.9 cm (56.65 in)56.65 in (143.9 cm) by 80.9 cm (31.87 in)31.87 in (80.9 cm), or about 11,647 cm² (1805 in²)1805 in² (11,647 cm²) of picture.

That's 40% more picture than a 55-inch, and the difference shows from the sofa. The 30 degree seat moves from 2.3 m (7.5 ft)7.5 ft (2.3 m) on a 55-inch to 2.7 m (8.8 ft)8.8 ft (2.7 m) here, so the bigger screen lets you sit farther back for the same view.

If your sofa is closer than 2.7 m (8.8 ft)8.8 ft (2.7 m), the 65 goes past a 30 degree view, and a 55 may suit the room better.