You can change the popup size under Settings → Size & Position.
In most cases, one value is set as the maximum, and the other is set as the minimum.
In the example image below, the maximum width is set to 350px. If the device width is less than 350px, the popup will shrink to fit the device width. If the device width is 350px or more, the popup will remain 350px wide.
The minimum height is set to 400px. As you add more content to the popup, the height will expand accordingly. Even if there is only a small amount of content, the popup will keep a height of 400px.
The popup size on desktop and mobile is independent. You must preview on mobile and ensure the mobile size is correct as well.
