wp_back/wp-content/themes/simple-theme/css/post-style.css

.max-upload-size {
  position: relative;
  padding-bottom: 25px;
  display: inline-block;
}
.max-upload-size::after {
  position: absolute;
  bottom: 0;
  left: 0;
  right: 0;
  content: 'Размер картинки: 1030х502px';
  color: red;
  text-align: center;
}