The width of the area containing the background image is the full width of the browser / device being viewed on.
You can change the height by adding some custom CSS in Appearance > Customize > Additional CSS:
.entry-header.with-image,
.archive-header.with-image {
min-height: 30rem;
}The above CSS is the default setting of 30rem (or 480px), so you could make it larger or smaller by changing this value, for example:
.entry-header.with-image,
.archive-header.with-image {
min-height: 300px;
}I’d recommend trying different height values until you find one that works for you.
