There are lotsa ways to figure max screen size for a given viewing distance... here's one of them...
A typical 1080p television, the most common resolution offered, has about 2 million tiny pixels for any give screen size. Each of these lights up separately to create the mosaic that becomes your television image. (You can learn more about screen resolution here). Since the number of pixels remains constant regardless of screen size, a small screen will have pixels that are smaller and closer together, while a larger screen will have pixels that are bigger and have the potential to be more visible as you approach the screen.
How close is too close? Assuming your vision is average, for a "Full HD" set with 1080p resolution, your maximum screen size can be calculated as the viewing distance (in feet) multiplied by 7.7. For example, for a viewing distance of 8 feet, 8 x 7.7 = 61.6 inches, which would translate today to a 60-inch or 65-inch screen. Go too much bigger at that distance, and you’ll risk starting to see the pixels.
"You say you've lost your faith, but that's not where its at.
You have no faith to lose, and ya know it" - Bob Dylan