The theme you are using has nothing to do with size of the post editor. You can't change the width of the dashboard post editor.

Yes your theme has nothing to do with your html editor in posting area in admin panel.

However you can change the height of that html editor box/area by going to
Settings > Writing and in the top line
edit
Size of the post box ??? lines
put 25 or more in the box for larger view.

Mohsin