ls command for files
du command for directories

Checking File Size

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

Checking Directory size

du -sh directory_name #Gives you the summarized(-s) size of the directory in human readable(-h) format
du -bsh * #Gives you the apparent(-b) summarized(-s) size of all the files and directories in the current directory in human readable(-h) format

For more information see man ls and man du