Width and Height

.card {
  width: 100%;
  max-width: 400px;   /* never wider than 400px */
  min-height: 200px;  /* at least 200px, grows with content */
}

img {
  max-width: 100%;    /* images never overflow their container */
  height: auto;
}

.square {
  width: 200px;
  aspect-ratio: 1;    /* height follows width */
}