When you use Visualforce page as Quick Action and modal popup window opens – it occupies 50% of container width. According to Create Object-Specific Quick Actions article
For a Custom Visualforce action, select the Visualforce page, and then
specify the height of the action window. The width is fixed.
Is there a "best practice" way to adjust width in this case using some code?
Best Answer
The width is fixed. You're meant to create a responsive design that fits in the margins. No css or JavaScript you write can change this.