$file->resize()

$file->resize(?int $width = null, ?int $height = null, ?int $quality = null): Kirby\Cms\FileVersion|Kirby\Cms\File|Kirby\Filesystem\Asset

if($image = $page->image()):
    // resize by width and height and echo the url
    echo $image->resize(300, 200);

    // adjust the quality
    echo $image->resize(300, 200, 80);

    // resize by height
    echo $image->resize(null, 200);

    // resize by width
    echo $image->resize(300);
endif;