As a general rule it usually makes a resonable job of creating messages and line lengths that work in most cases. However there are times where replying to some that have been formatted in a particular way you get odd results.
To reformat the message, simply hit CTRL + R and then all will be well again.
If you use the reply button on the top tool bar then it works as you would expect (i.e. does a reply to group). The one on the message pane area is a bit more "specific" and duplicates some of the options on the main message menu.
Na, that's someone giving instructions for a years out of date linux version...
If you really want to find the setting (and there is not much point since the default is 72), you need Tools Menu | Options... | Advanced button | Config editor button. Then type wrap into the search box and you will see the setting.
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