In case anyone is interested, I did a little test.
At 240 pixels, my database is 95 mb
At 320 pixels, my database is 120 mb
At 480 pixels, my database is 217 mb
In addition, on my Galaxy Note 3, the 240 pixel covers didn't look that different to the 480 pixel covers in cover view or list view. If I set the cover size to extra large in details view I can see a pretty big difference, but even then its not so bad at the lower resolution.
I don't have my tablet at work, but I imagine it would make much more of a difference there.
If anyone is curious, I tested this with these covers:
http://tinyurl.com/plrtd2l which I found yesterday and replaced the default covers in my library with.