I need a custom CSS to size the page when it opens to the form width

I need a custom CSS to size the page when it opens to the form width so it does not open in a full page...

I'm sorry, but would you mind explaining this further? What is the form in question? We have a feature where a form opens in fullscreen, but it's available in our Card Form layout only. Your last 3 edited forms are in classic layout. Please clarify.

Thanks