Are you looking to change the content itself ("hard" line breaks are
stored in the document) or do you want to change the way the window
displays long lines? (soft line breaks)
Some relevant preferences options concerning line breaks:
"Editing: General"
-- "Use 'hard' lines in soft-wrapped views" (checkbox)
-- "Soft wrapped line indentation" (radio: Flush left, First line,
Reverse)
"Editor Defaults"
-- "Soft Wrap Text" (checkbox): Wrap to (radio: page guide, window
width, character width)
"Text Files"
-- "Default line breaks" (radio: Classic Mac [CR], UNIX [LF], Windows
[CRLF])
-ives