Quick View Modal Box Width?

Hello @devilmakq,

Yes, to achieve it, you need to write custom CSS. So, you can learn how to use Chrome Developer Tools or Firefox Developer Tools to help you see and test changes to your CSS:
https://developers.google.com/web/tools/chrome-devtools/
https://developer.mozilla.org/en-US/docs/Tools

CSS Tutorial: https://www.w3schools.com/css/

And for responsive sizes, put your CSS on custom screen size:

@media only screen and (max-width: 480px) {
    /* put your custom CSS here*/
}

Or use between sizes:

@media (max-width: 960px) and (min-width: 481px){
    /* put your custom CSS here */
}

For more information about media queries, please read this article:
https://www.w3schools.com/css/css_rwd_mediaqueries.asp

Example:

.owp-qv-content-inner {
    max-width: 960px;
    padding: 30px;
    border-radius: 6px;
    overflow: hidden;
}

I hope it helps.
Best Regards