Custom CSS:

.Skin .Matrix .CS td input { width: ....px}

Hi Ahmed. I am not sure what you mean. Do I copy the text you put in the answer somewhere in the question? Sorry I am not familiar with any script writing.

You'll have to read the support pages on styling surveys.

ahmedA is referring to this Qualtrics Support page. Inside the survey, go to Look & Feel (I always go to the old editor since our theme is severely outdated) -> Choose Style. You'll find a "Custom CSS" textbox. That's where you want to paste in ahmedA's CSS to find and change your input textboxes' length. It'll look like this:
.Skin .Matrix .CS input {
width: 150px
}
Just change the number to suite your needs.
*Note: this will change it for every text input in your survey. If you want to get more granular with your textbox sizes (like just this question or different statements to have different sizes inside a question), I can give you a JavaScript snippet I build and use.