How do I display the file size and format of attachments on a node templates?
There's a module which provides a Twig filter for that: Format Bytes

In twig when you get the value of a file, the size is in simple bytes and if you're intending to show it to your end-user, it's not user-friendly, this simple Twig filter helps with that and renders the file size in it's respected: 'B', 'KB', 'MB', 'GB', 'TB'

Example usage:
{{ node.field_attachment.entity.field_file.entity.filesize.value | format_bytes }}

The alternative would be to do the same formatting that module does in your own preprocess function for the template.