If you're writing a game with lots of resources you probably have to give it a higher value. Choose the size depending on your game. If you're just developing an usual app which doesn't require too much or no external memory you even can create just a 10MB sd card. Keep in mind that eclipse creates a sdcard.img which has the size you have entered, no matter how much is actually stored on the virtual sdcard.
I'm not sure but I guess that the size of the sdcard also has a little impact on the booting t time. So 8GB is too much, you'll never use it.
I always create a 10-100MB virtual sd card
Hope it helps a little
