Why do the images only display in original size, not my custom size?

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The reason is WordPress makes different sizes of images(i.e thumbnail,medium,large & full) when you upload the image.

If image is already uploaded and after that you declare the new size then that image size is not going to be there.

So you can solve this by two ways

  1. you need to regenerate the thumbnails using some plugin like Regenerate Thumbnails
  2. Upload the image again

All the best