I think the plain image will be whatever the original size was, but the rendered images, which is what Delicious Library will always use as long as they exist, will always be a certain size.
So, no, there is no optimal size, other than that adding something bigger than the biggest cover you can make in Delicious Library is just a waste of hard drive space.
Definitely make sure images are 72 dpi, though I actually fixed this in Delicious Library 2, I doubt it will be back ported.
I think both your requests will find their way into Delicious Library 2.