Command to know the size of a file in Linux

To know what is the size of a specific file in you Linux server, you can call one of the following commands :

ls command
du command
stat command

ls -l filename /* Size of the file*/
ls -l * /* Size of All the files in the current directory */
ls -al * /* Size of All the files including hidden files in the current directory */
ls -al dir/ /* Size of All the files including hidden files in the ‘dir’ directory */