Good question!

My guess would be to think of the maximum width of text fields the same way we think of bodies of copy. That is to say, your text box shouldn't be longer than 50-75 characters per line (https://baymard.com/blog/line-length-readability).

Another way to think about it is that the length of your textbox should indicate to the user what it is for. For example, a 'zip code' should only be as long as 4 or 6 characters depending on your country.

Hope this helps!