For seasonal calendars, the lead time may be even longer. Here you will probably find yourself tailoring your shoot to the particular needs of the publications you are submitting to. That is enough reason alone to stick to the standard size. Unless color prints are custom printed, they will seldom achieve the brilliance of a good slide. For magazines, it is still the norm to submit color slides, not prints.
Slides are more brilliant, and therefore more impressive.
This is good news for the cost-conscious, because slides are much cheaper to shoot, develop and duplicate than color prints. A little first-hand research will tell you a lot. Look at about three weeks of theater coverage in the New York newspapers and note the kinds of photos you see. It is interesting to note that dancers' feet are usually, but not always, visible in production shots. On the calendar pages, you will notice that photos are cropped tightly, with the lower body usually not shown.