I’ve just tried looping through an acf_file field using your code sample above.
The url and type fields works fine but the size field returns nothing, strange…
I just tried with the exact same settings as you (I have the same plugin versions and my file field is set to return an array) and using the template that Eliot shared above worked for me. I uploaded a 3 MB (3,275,915 bytes) PDF to a CPT and the output was this:
Is there anything else going on on your site that might be causing this? Are you able to replicate it on a fresh WP installation? Let me know what you find.
Just for the sake of testing, I tried working with an ACF file field from an options page by adding from=options to my Loop and Field tags and it worked as expected, giving me the file size. Let me know if your testing reveals anything that might be causing this issue on your particular installation.