There are about 61 cubic inches per liter. Thus, 1700 cubic inches would be roughly 28 liters and 1200 cubic inches is roughly 20 liters. That is in the range of common sizes. Have you tried to ride around town with most of your weight loaded on a back rack? You will have to decide what you are comfortable with when it comes to handling. Some bikes handle better than others with most of the weight on the back rack.