The new menu appears tight on the right hand side. It seems it is almost as wide as the item list with the categories. Consider using the 2/3 rule where the right hand side of the menu is 2/3 of the list area. This is a good rule of thumb to balance UIs and is used in graphic design a lot for the same purpose. I think that would help the menu feel weighted properly giving more space where the most important content is located.
You mean double the width of the left list pane, right? Making the right pane 2/3 of the width of the left list would make no sense, as the right pane is the content area. I'll assume you mean double the width of the left list pane. We had that for Plasma 5.20 and 5.21 but it was removed for 5.22 in favor of the tighter layout we have now. I agree that making the right pane wider would look nicer.
This is done in 5.26 now, and in addition, the popup size is nor configurable so you can change it to whatever size you prefer if you don't find the default size to be ideal.