My personal philosophy is that it depends on the title and/or the image. If you look at the
Thistle and Flame cover in my siggy, it's one of the biggest titles I've gone with, and I did it because I could--the image itself was simple, and the font I chose didn't interfere with it. So basically, start with a tentative cover image, see what you can work around. I'd make the title as big and as readable as you can, without taking away from the picture. Sometimes you will need to make the title as small as you can (while still being highly readable in thumbnail) in order to show enough of the cool picture. Just depends.
Personally, I'm bored by covers that are all words or boring/obscured pictures, but that's just me. I skim down a search results list, and stop at the first cover that catches my eye (by being well-done, by being related to a genre I like, and by the title catching my interest). The title needs to at least be big enough to read the title in a fraction of a second, the pic needs to catch my attention and be "understood" in a fraction of a second, and the author name needs to be readable at a second glance at least.
This is all just my opinion, based on my own preferences and habits.
