A few approaches that are often used would be to crop the image, by clipping off extraneous parts or resizing an image by squeezing or extending the sides.

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It would often be best to choose a size that would work best for the output. Other, more interesting approaches, would be to take glances of the image using an RNN, which would allow you to make the input any size [1 5, 2 5, 3 3]. A more naive approach would be to add 0 padding to all of the extra space in the image; however, this would take up a lot of memory.