In reply to Cats: the most common ones i do at work are portrait, Because both landscape and portraight pictures will fit the same layout, Because when you add the calendar bit it effectivly becomes landscape where your image goes, and a landscape calendar becomes 'squished' landscape.
Size wise, Apart from personal preferance a lot of 'local charity' calendars that are printed A3 would look massively better on A4 because of poor quality images, So make sure your art work is upto scratch.
If you getting it digitaly printed why not have two sizes? Although you can't beat good quality Litho...
Good luck with your calendar, and if you want a quote for anything post press (cut collate punch bind bag shrink etc...) I'll do you it just above cost.
Oh my personal preferance is to have two calendars, landscape for pretty pictures and usually and A4 portraight for writing stuff in.