How to display file size information using media

Hi @marcjana_84,

You can try using below solution,

1) Add this in .theme file in custom sub theme

<?php
function HOOK_preprocess_file_link(&$variables) {
  if (isset($variables['file'])) {
    $variables['fileLinkDisplay'] = [
      'fileSize' => $variables['file']
        ->getSize(),
      'fileSizeFormatted' => format_size($variables['file']
        ->getSize()),
    ];
  }
}
?>

2) Add this to custom sub theme
-> themes/custom/THEME_NAME/templates/field/file-link.html.twig

{{ fileLinkDisplay.fileSize }} and {{ fileLinkDisplay.fileSizeFormatted }} will be available in your template.
In twig file we can use as per below:-
<span{{ attributes }}>{{ icon }} {{ link }} {{ fileLinkDisplay.fileSize }}  {{ fileLinkDisplay.fileSizeFormatted }}</span>

Thanks