Paste the below code in the matrix JS onReady function and adjust width value as required.
 jQuery("#"+this.questionId+" inputttype='text']").css("width","12em");

rondev , could you pleas tell me how to assign different width to each column?
For example, the text entry of the first column width = 12, that of the second column width = 5, ...

https://www.qualtrics.com/community/discussion/comment/29833#Comment_29833line with c4 is first column, c5 second and so on:
jQuery("#"+this.questionId+" .c4 input[type='text']").css("width","12em");
jQuery("#"+this.questionId+" .c5 input[type='text']").css("width","6em");
jQuery("#"+this.questionId+" .c6 input[type='text']").css("width","3em");

can you clarify where to add the custom text w/in the survey platform?