IIRC calibre OPDS acquisition feeds include fie length and mtime as attributes.
For example:
Code:
<link type="application/x-mobi8-ebook" href="/get/azw3/1356/test_library" rel="http://opds-spec.org/acquisition" length="3354704" mtime="2023-11-18T02:16:51+00:00"/>
<link type="application/vnd.openxmlformats-officedocument.wordprocessingml.document" href="/get/docx/1356/test_library" rel="http://opds-spec.org/acquisition" length="3039862" mtime="2023-11-18T02:16:51+00:00"/>
<link type="application/epub+zip" href="/get/epub/1356/test_library" rel="http://opds-spec.org/acquisition" length="2657477" mtime="2023-11-18T02:16:51+00:00"/>