This might be a bit of an 'out there' idea.
Set the width of the text box, then in the Placeholder option instead of carriage returns you can space out your words to 'pad' then to the next line.
Now, Spaces won't work, Infopath just removes all of them leaving only one. So try holding ALT and typing 255 on your NUMBER PAD (must be the number pad) and a let go of ALT.
A 'Space' will appear in the Placeholder box except Infopath won't remove these ones. Just do a few of them, and then copy and paste them as needed.
Someone may have a better ALT code to use, but 255 is the first one that worked as I wanted.
This seems to have the desired affect, but is a bit fiddly.
The reason for setting the width first is clear, because if the width is changed, then you'll need to readjust all the spaces.
Good luck.