The book title and cover is your first introduction to the reader, treat it as you would your business. After all, you want to make a great impression on those casually browsing.
Small is good.
Your ebook cover on Amazon could be seen at its’ smallest at around 80 x 56 pixels. This is not a lot of screen real estate you’ve got to play with. Of course, it does get progressively bigger if a reader chooses to buy your book. Your cover has to look good at it’s smallest and up to 1024 x 768 on an iPad or 1280 x 800 on a Samsung Galaxy Tab. Hard to believe so much could be asked of one little cover.