It's simple. Use ls command for files and du command for directories.
Checking File Sizes
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 */
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