4K describes resolution, not physical size
A 4K television normally has a 3840 by 2160 pixel picture. That resolution can appear on several screen sizes, so there is no single diagonal that makes a TV properly 4K. The useful question is how much of the detail you can appreciate from your seat while enjoying a comfortable overall view.
Why distance changes the decision
As you sit farther away, fine details occupy a smaller part of your field of view. Increasing screen size makes those details larger, but also makes the whole image more immersive. Moving a seat closer can have a similar visual effect without changing the screen.
Sony publishes a 4K minimum viewing-distance guideline based on 1.5 times the screen's vertical height. Treat a minimum-distance guideline as one reference, not a requirement for the best sofa position. Comfort, content and the room layout still matter.
Compare real content
Look at the television with the material you expect to watch: streamed films, live sports, games or older broadcasts. The source quality and picture processing affect what you see. A high pixel count cannot make every low-quality source look like native high-resolution material.
Separate the picture choice from the mounting choice
Choose a size that works at your viewing distance, then check the actual TV's width, height, weight and VESA pattern. Allow for the soundbar and screen movement. Mount specifications do not change just because the television is labeled 4K.
Use our screen-size comparison table to mark out candidate sizes. If you are replacing an existing TV, send the old and new model numbers when asking whether the current bracket can be reused.