By Percentage

Using the Expand Image operation, you can opt to scale up the image using a percentage value. When the percentage option is checked, the AI will expand your image based on a percentage of the total resolution of the selected image. The amount of pixels that the AI will generate will be evenly distributed along the X and Y-axes.

NOTE By default the X and Y-axes are linked. In order to expand your image on only one axis or to offset the distribution of generated pixels, you can unlink the axes in the Expand Image dialog by clicking the lock icon.
NOTE At the bottom of the Expand Image dialog, there is a disclaimer advising that no image can be expanded beyond 5k resolution.

If your image is 100 px and you set the value to 100%, the AI will generate 100 px to add onto your original image. This will double the size of your image to 200 px total. If you set the value to 50%, the AI will generate 50 px to add to your original image. This would enlarge your image to a total scale of 150 px.