In the old days, our screen adapters could handle a line of 80 characters of text in 25 lines total (the char of 80×25 would have scrolled the screen to a new line).
Today most cards support a lot more of text modes then 80×25. For example we have support for VESA that allows us to have a bigger resolution range, not to mention GUI environment that we develop in today. But in development we kept the principle of 80×25 for the length of a line.
People so got used to it that a code or a comment does not considered to be readable if it's not 80×25.
Even though this subject was talked to death about it, no one have come out to a new rule of thumb on how long a line should be.
What is your optimized screen resolution and why ?