abcPDF will only letter wrap if:
- wrapping is on
- There is not enough horizontal space to fit one of the words being set
- There is vertical room for another line in the active rect
These conditions therefore amount to there being plenty of vertical room but not enough horizontal room for some particularly long word. So a heuristic for finding the correct font size would be to test horizontally first, using only the longest word from your string, in a temporary rect that shrinks to one more than the font size as you reduce font size; then, once you have the right font size to avoid letter wrapping, go back to testing with the original rect and full string, continuing to decrease font size until truncation completely disappears.
This will get much hairier if what you're trying to set is actually HTML with variant fonts or sizes; but for plain text in a single font and style, it should be ok.