If you are using the Flexible Sizes Print size option, these smaller sizes will not be displayed to customers unless the aspect ratio of the image is the same as the print size. For example, a 4"x6" print size will only be shown if the aspect ratio of the image being sold is 2:3.
This is because we print these smaller sizes from pre-cut photo papers and we are not able to trim off extra white space as we usually would when using the Flexible Sizes option.