Most outlets have requirements or recommendations for minimum cover sizes...
Apple = at least 1,400 pixels wide required
= 625x1000 minimum, 1563x2500 recommended
B&N = 1,200 to 2,000 pixels high recommended
Smashwords = Same as Apple
With all the higher resolution devices it's nice to have high rez covers (speaking as a consumer). The biggest thing for an author/publisher to keep in mind is that higher rez is nice, but some distributors (such as Amazon) charge a fee based on the size of your file.
This post on cover size from Smashwords might be helpful..