When you are adding or modifying a photo, you can choose which print sizes and options are available for sale for that photo.
Simply click the checkbox next to each print size that applies. Be sure to select the print sizes that have the same aspect ratio as the photo. If there's no print sizes with matching (or very close) aspect ratios, then you may need to add a new set of print sizes for that aspect ratio.
If you select a product category, such as a print size, but you don't click any of the display options underneath, then ALL those display options will be for sale as well. This is for convenience, so that you don't have to click every single repeated item.
What if I add a new print size or option? Do I then have to modify every photo individually to apply the new size/option?
Thankfully, no! There is another much faster way to match up the photos and prints in this scenario: you can batch assign multiple prints sizes to multiple photos all at once! Read more about batch assigning.