Posted by Stephen Fry on his twitter account.  WHAT A TOTALLY AMAZING IMAGE!!

Cut and paste from Yahoo answers....

If the earth was the size of a BB, the sun would be a couple inches bigger than a basketball. YV Canis Majoris would be more than a mile in diameter.
This should be considered with the understanding that it is difficult to measure the size of such big and distant stars. Our estimate of the size of YV CMaj may be off, but the scale I gave you is probably pretty accurate.
There are several scale demonstrations on the Internet, but I like this one best. It includes VV Cephei which is probably just a little bit smaller than YV Canis Majoris. I believe the size comparisons are among the more accurate ones.
http://www.youtube.com/watch?v=2l4leBHRvI4&feature=related