It would be great if the single-line string field would automatically grow and shrink as text is added, similar to the multiple paragraph field when set to the simple textarea presentation option.
I realise that in practice this would mean using a <textarea> under the hood and suppressing line breaks and return key presses, but it would add a lot of usability. Often you want an unbroken string of a fair length (say for a lede on an article, or a quote). It’s harder for editors to update the text when they can’t see it all at once.