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Open up an image and set the desired image size.

Go back to image you are enlarging.  In other words, the original image.  Use cmd+T (transform) and draw a square across the area of the original image.  This will highlight the image and enable you to copy it.  Go to the second image and paste the first image over it.  Then cmd+T and pull in/out the corner handles of the image.  You will now be able to enlarge the image withing the borders of the second image's canvas size.