You see that I have mine at a perfect computer and email viewing size, 640x480. Anything close to this is acceptable. It took me two tries to get it to this size. I started with 50%. This was not enough. So I clicked Undo (found under EDIT) and started over using 40%. My second try just happened to be the right size. Write this down if you need to. If you are resizing pictures from the same camera, the percentage you find should always be correct. Obviously, you will need to start over when resizing pictures from a different source.
Remember, always use the SAVE AS, not the SAVE function or you will lose your original. This size is too small for a good portfolio shot. Its great for posting on the Internet and sending pictures by email to Mom or Sis. They appreciate the smaller pictures also.
As everything else goes, its really quite easy. It just takes practice.