When I'm uploading file from lightning out (apex page) and attach it to my record in Notes & Attachments related list salesforce doesn't show file size. It shows only when I'm uploading from salesforce page. How to show file size?
Bodylength - Size of the file (in bytes) of the attachment.
Unfortunately, You cannot add this field on notes and attachment-related list as this is not customizable.
The workaround is you can create a simple custom Lightning Component to show attachment as a table.