Thanks for your answer kovidgoyal - and let me take the chance: thanks a lot for your great software calibre!
As I wrote I use the mentioned user style:
body { max-width: 50em !important }
No other user style at the moment.
What do you think about the idea to allow to set the width in the settings to a relative unit like "em"?
May be I should explain, why I prefer a relative unit.
With a portable device I have a typical distance between hand and eye.
When I use a big display from my desktop pc that is different. Sometimes I lay back in my chair, than I need a bigger font size. Than I sit straight and near the display. Than I need a smaller font size.
That is a typical use case, I suppose.
I don't like it at all, when there are to much characters in one line.
An the only way to set it to a maximum is a relative length unit.
May be you like my suggestion