you'll need to find the end of each line in your multi-line
textfield. to do that, create a new multiline textfield, get the
textformat of the original, as well as, its _width and assign those
properties to the new single-line textfield.
now start assigning the text property of the original
textfield, one character at a time to the new textfield. when the
maxscroll property of the new textfield exceeds 1 start removing
characters from the end until you encounter a place to break (like
a space or perioid, comma, question mark etc). that's your first
line. continue as above for each line.