Like Deardorff38, I print the image format dimensions, not the paper dimensions. The border size varied when I printed in the chemical darkroom. Today, printing digitally, I've standardized on 6x9" on 8.5x11 paper and 12x18" on 13x19 paper.
As I understand it, historically the border was there to protect the image area when handling unmounted prints. Paper corners could become dogeared and less than perfectly clean fingers could leave residue on the image without a border.