I would first try deleting the question entirely and re-making it. 

If you see these little lines in the bottom right of the text entry box (screenshot below), then you can use your mouse to manually re-size the text box, but, I don’t think the lines appear when you add a text entry box to a regular response option (i.e., through the menu that you’re showing in your original post).

If you can’t manually re-size, you could try adding some custom code, e.g.,:
jQuery("#"+this.questionId+" .InputText").css("width","100%");
(source)

or

jQuery("#"+this.questionId+" .TextEntryBox").css({"width":"100%"});

(source)

I wasn’t able to replicate the issue in my Qualtrics account, so I couldn’t check to see if that code will work. What type of question were you trying to add the text box to? Is it a problem in your whole survey or just with this one question?

Thanks for your reply, ​@lizb311. The issue happens when using the allow text entry option for multiple choice questions. It also seems to only happen when using the new survey look and feel. I am not having this issue with surveys using the old survey taking experience. 

Ohh. Yes, it looks like we cannot resize text boxes with the new survey taking experience right now - it’s on the list of unsupported features. You may need to revert to the “old” experience right now to be able to resize that text box. 

--- 

I confirmed that adding jQuery("#"+this.questionId+" .TextEntryBox").css({"width":"10%"});  to a regular survey (not the new layout) to the question Javascript in the Add On Ready section works. 

As far as I understand it, in the new experience, you have to call jquery first in Look & Feel → General → Header → source. I tried this by adding the following code:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js"></script>

However, even after doing that, I wasn’t able to change the size of the text box using JS in the question. I tried changing the instance of jquery I’m calling, and tried editing the JS in the question a few different ways. But no luck so far :( 

So I’ll be curious to hear if anybody else has managed to call jquery successfully with the new survey taking experience - and if so, if they’re able to figure out the updated code for changing the text box size. I haven’t figured it out quite yet, sorry :( 

No problem! Thanks so much for your help.