html,
body[data-widget="300x250"] {
    width: 100%;
    height: 100%;
}

body[data-widget="300x250"] {
    display: grid;
    place-items: center;
    overflow: hidden;
    background: transparent;
}

body[data-widget="300x250"] .widget-container {
    display: flex;
    flex-direction: column;
    width: 300px;
    height: 250px;
    max-width: 100%;
    max-height: 100vh;
}