“Get Filesize Shortcode” is a simple shortcode to get filesize of a file( eg. PDF, JPG, PNG … ) with a human readable format, using the largest unit the bytes will fit into.
Now added Get filesize block to display file link with file size.

Usage

Put [filesize]http://yoursite.com/path/to/file.pdf[/filesize] anywhere in a post.

Or you can use url attr instead. [filesize url='http://yoursite.com/path/to/file.pdf']

Also you can place <?php echo do_shortcode('[filesize]http://yoursite.com/path/to/file.pdf[/filesize]'); ?> in your templates.

As to “Get filesize” block, search ‘get filesize’ in Text block category and insert it.
Set file title and file url, and it generates a text link to the file with file size afterwards.
You can toggle Preview/Edit by clicking Preview/Edit button in toolbar of the block.

Note

File must be in your server
With files larger than 2MB, it might return different number due to the spec of filesize function of PHP.