I tried to execute the data size command with applications, folders and files, but every time it returned 0 bytes.

tell application "Finder"
    set appPath to POSIX file "/Applications/Slack.app"
    set appSize to data size of appPath as application
    display dialog "The application size is " & appSize & " bytes."
end tell

However, if you look in details of the app it shows 444,6 МB.