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We use something in the neighborhood of 800(w) x 1200(h) in jpg format. That seems to keep most distributors happy, and displays well. (Kindlegen, for example, will whine if the cover is too small.)
IMHO it is not necessary to attempt to design a cover that "looks good" in both landscape and portrait orientations. At sales outlets, cover thumbnails are going to be portrait, and people mostly "expect" that aspect ratio. Once they buy the book, of course, they can display it however they want, but at that point admiring the cover in landscape mode is probably not what they are most interested in.
In other words, unless you are designing some kind of fixed-format "coffee table" book that is intended to be viewed in landscape from the get-go, I wouldn't worry overmuch about landscape mode.
Albert
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