The normal way of designating print sizes for each photo is to add or modify each individual photo and select each applicable print size. But what if you add another print size later on... do you then have to modify EVERY single photo again? NO! There is a tool that makes this very quick and easy: batch assigning.
Say you've just added a new print size, and want to apply it to every photo. Go to PRINTS > BATCH ASSIGN and on that page you have some powerful tools for assigning which photos should apply to which print sizes and products.
On the left you'll see a selection list of all your photos - you can click next to each photo you want to assign to any print sizes or products that you select on the right side. There are also various filtering tools to help narrow down the photo list to only the photos you're looking for.
This is probably the most complex and powerful tool in the admin, so make sure you read the directions there and understand what you are doing! If you are unsure how this works, please contact Jack for help.