Use the below code in the onReady function of the matrix question (change the % as required)
jQuery(".SkinInner").css("width","65%");

Thank you! But it does not work.
Why the content sticks in the middle? I want to expand the area to see other items of 7 point Likert scale.

Modify your CSS media rule to this:
@media only screen and (min-width: 1200px) {
.Skin .SkinInner{width:1200px;max-width:95%}
#Questions{min-width:95%;}
  #SkinContent{min-width:95%;}
}

it works! Thank you.

Hi TomG I used your above advice (October 2020, modify CSS media) for the same kind of problem (survey was way too narrow, all centered with a lot of blank white space on both left and right). Your code advice above solved a huge part of my problem too (thanks!), but while now I have no blank white space to the left of all my matrix tables (I have over 300!), I do have a lot of open blank white space on the right side of the screen. The scale point statements in my columns are text wrapping several lines down. Any additional feedback aside? The idea of needing to manually adjust every column x 300+ questions seems time consuming. Thanks!

Add this line to your question JS:

this.getChoiceContainer().querySelector("tr").children[0].style.width = "10%"

https://www.qualtrics.com/community/discussion/comment/33423#Comment_33423There is probably a CSS rule constraining your Matrix to a fixed maximum width. Use the Inspect feature of your browser to find it, then add a rule to your Look & Feel to override it making the maximum width 100%.