What's the biggest TV size I can fit on this wall, and which are some decent brands for the same?

Not too sure about brands but according to this, somewhere between a 42 and 49 inch screen is the max you could probably fit on that wall.

AMG said:

The wall (on which the current TV is mounted) is 40 inches wide.
So I'm kind of real-estate limited.

Viewing distance is 10-12 feet.
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Best option is to get a carpenter's measuring tape and match it diagonally across the TV.

I guess you can arrange 55 inch TV on a wall mount by swiveling it a bit. So right side would be closer to the wall behind while left side would be a bit farther away.