Yes, you'll have to calculate the size of the print - I agree the software should do that, bou it doesn't (you'd need a RIP for that).
So, take the 13" width and allow for some margins (1/2" eg), your image is 12" high, so you'll know the length accordingly and add those margins again (you may use the image size dialog, as long as you don't resample the image unadvertantly).
Set a custom paper size in the printer driver with those dimensions, and you're good to go.
Of course, you'll have to set one custom size for each image...