Submerge the entire plant in a bucket of water to which a few tablespoons of a transplant solution such as Vitamin B-1 have been added. It should soak at least 12 hours to rehydrate the root system after several days or even weeks of exposure to less than perfect moisture conditions. If you are delayed in planting, it can soak for 3-4 days, but change the water daily to discourage bacterial growth. The size of the hole seems like 'overkill', but it will pay off in spades in future performance. The hole in which you plant is the most important aspect of growing good roses from bare roots.
Depth is for drainage.
Other than anchor roots which aren't the main nutrient suppliers, most roses grow only about 12 inches deep. However, they require lots of water and won't tolerate wet feet from lack of drainage. If you are confident you have loose, draining soil to a depth of 2 feet then you may dig to only 18 inches. If you aren't confident, fill the hole with water. Don't compromise on the width.