Agree with jmm1837. Once you tape up the various sizes you will probably see that the larger screen will look shoe-horned into the space - which isn't a good look IMO. If there is no space around the screen, or very little space, it will emphasize how small/short/tight the wall appears visually. Even if the wall isn't small - it's an optical illusion. Give your TV screen lots of space on the wall.

Agree with both of the above. Blue tape is my friend - cardboard too! sometimes both

Treat it like any piece of furniture on a skinny wall; 25% of the piece's width should be the minimum amount of exposed wall flanking it on either side. I think the TV sitting on the floor is about the biggest I'd go on that stairwell wall. If there is a closet under the stairs, the pros often hide all the gear in there. While you're still under construction, lower that connection so your TV will be at the correct height for viewing.

I would go to a store which carries the tvs you are interested in, measure them, and get the largest one with a width less than 57".

The bigger the tv, the smaller the room appears to be. Could even become awkward. To me the size of the tv, that stands on the floor, is about the right size for that wall.