@prasannas5456   does this article 'Modifying presentation options of the optimized table layout' help?  

EXTRACT:

In the Table Layout:

1. In the Width of content list, define whether the width of the layout columns is set as a percentage or in pixels:

   • To set minimum column widths that expand when the content exceeds the minimum width, select Pixels (fit content).
   • To set column widths as fractions of the total width of the layout area, specified in pixels, select Fill (100%).

2. To define additional layout behavior, select any number of the following check boxes, which represent specific personalization settings:

   • To display the container headings, select the Display container header and titlecheck box, and then configure the headings in the Header settings section, which opens when you select the option.
   • To make your grid adjust to screen width changes, select the Responsiveness check box.

     Note: The responsiveness option is not present when you enable the Allow Group by (Additional settings on the column) option on the Operations tab.

     Tip: When you enable responsiveness, you can specify which columns users can see in the table when the available screen width is reduced. For more information, see Arranging column visibility by importance.

There is also information available on Pega Academy  Responsive UI and tabular data

There are also two archived support articles that might help:

Column headers are not aligned with data in expanded tree grid

Column alignment distorts after Fixed Size with Freezed Header

For Cosmos, also see Modifying presentation options of the non-optimized table layout

Also note that there is an enhancement logged: FDBK-75924 and US-96100.

This requests changes for the requirement to Freeze Repeating Grid -> Header and First Column. Grid Width and Height should be flexible/responsive and should not be fixed (should auto fit based on screen resolution).  For now please see available documentation and re-size columns manually as required.

You may also want to follow another user who posted a similar question Table width does not change more than 1020 if Freeze header is used .

Also please follow the advice given by our UI expert in this post How to freeze the first column and the header of an optimized table