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?
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I think you can achieve it by doing some CSS adjustments. Check the following blog which has an example with source code.