Shorter lines tend to be more comfortable to read, just not too short. At least that's my preference.
The automatic line breaking prescribed by the SSA format, inherited by the ASS format, and implemented for that in VSFilter and libass, specifies that when a line becomes too wide to fit between margins, it gets broken in the middle, such that the upper line is longer if they can't be equal length. That usually gives a reasonable visual presentation, but manually breaking lines to suit the text can certainly improve on it.
There are also alternate line breaking modes, to choose that the upper line should become the shorter one, or to do a simple overflow break and fit as much text as possible on the first line. Or you can disable it altogether.
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