How can I see the size of files and directories in linux?

Use ls command for files and du command for directories.

Checking File Sizes

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

ls command will not list the actual size of directories(why?). Therefore, we use du for this purpose.

Checking Directory sizes

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

Including -h option in any of the above commands (for Ex: ls -lh * or du -sh) will give you size in human readable format (kb, mb,gb, ...)

For more information see man ls and man du

There is du command.

Size of a directory and/or file, in a human-friendly way:

$ du -sh .bashrc /tmp

I memorised it as a non-existent English word dush.

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--apparent-size command line switch makes it measure apparent sizes (what ls shows) rather than actual disk usage.

Use ls -s to list file size, or if you prefer ls -sh for human readable sizes.

For directories use du, and again, du -h for human readable sizes.