I found this a long time ago, but I just came across it again when going through bookmarks.
It's probably more of a resource for novice astrophotographers, but how and ever...
It shows the size of some of the more photogenic DSOs compared to the moon, which I think
is a reasonable standard for most people to compare size to:
http://www.cloudynights.com/photopost/data/500/9328Astro-poster_lr_...
Kudos to Clownfish over on Cloudy Nights for making it.