How do I make a photo file size LARGER?

Hello - I want to be able to make a photo file larger -- say, take a jpeg file that is 8 k and make it 40 k. I have tried exporting the photo, but the option only lets you make your photo/file "NO BIGGER THAN" a certain size. That does not do what I want it to do, as it does not make the file a larger size. What's the trick? How do I do this? (Aside from purchasing an app like Photoshop or Aperture, that is...)
Thank you,
Nick