Hi Jane, according to me, 1920x1080 or how much you need height and 72px

Jane, this depends on the purpose, generically I have always used 1200 - 1920px wide, but A.Kris is quite right, it depends on the purpose. If it is in a slide show or image that can never be displayed at greater than 400px, then 400px is most appropriate.

Whilst I still resize/resample my images to a reasonable size (silly to use 5000px wide images), much of this requirement has now been removed with the release of Optimizer.