Most people who come back to swap a TV wish they had bought a bigger one, not a smaller one (sample guide).
Measure the distance from where you sit to the wall in metres. For a 4K TV, a comfortable screen diagonal is roughly two-thirds of that distance: at 2.1 metres, a 55-inch TV fills your view without making you turn your head.
Check the width as well: a 65-inch TV is about 145 cm wide, so measure the console or the wall space first. Our size guide on the TVs page draws every size to scale.