To determine the size of a file called /bin/grep for example.
Use the du command below
du -h /bin/grep
or the stat command which provide a lot more info
Use the du command with -s (summarize) and -h (human readable format) option to summaries the size of disk space taken by current directory.
If it to large and you need to reduce it but keep the quality there is not a lot you can do. You might want to edit any images and see if you can reduce the resolution a bit or the number of colors in the color pallet.