Well I had a go, but I don't think I totally understand what he wants to do.
Funnily enough the thought of putting a picture of outside on the TV to make it look like a 3rd window had crossed my mind...
Ha! Great minds...
FYI, we have just upgraded our TV in a similar-sized living room. We went for a 50" Samsung 6200 and apart from one dead pixel and one slightly lazy one (not that noticeable unless it is a white screen for an extended time, which, never happens really. Plus we got it at a great price by using deal-offers together), it is very good.
I came to the same conclusion as you regarding 4k TVs. We did have a 32", but as you say, the bezel sizes have got smaller, so that 32" probably worked out to 36". We wanted something that would be relatively future-proof, so if we moved and got a bigger living room, we would not have to upgrade again.
Watching movies on it is excellent as it provides a feeling of involvement, similar to being at the cinema with the big screen. Something to consider when watching a movie and having a smaller TV is the aspect-ratio and the horizontal black bars. You may have a 50" TV, but when you are watching a movie, the black bars reduce the image to the correct aspect ratio, so if you measured just the picture showing, it maybe a 42-44" viewable area. So, if you go for a smaller size, you are reducing again the viewing area for movies.
I guess if you consider why you are upgrading and come to the conclusion for entertainment, it makes sense to 'immerse' yourself in that entertainment, so 55" should be great. (Unless you're Art and go for a 1000" projector and become part of the movie
) .
Another point is that TVs have become very well designed in terms of aesthetics, they are now not just to watch TV, but they are part of the furniture. To some extent, with a good-looking, large screen, you improve the look of your living room, even when it is off (IMHO I guess).
The 55" will be epic!