Click here for a gorgeous, interactive science website on the relative sizes of things in nature. If you are a science teacher, this is not just a recommendation. It’s mandatory you explore this resource for your students if you haven’t already.
The site is designed by Cary and Michael Huang. Phil Plait commented on it in February. His comments section contains some interesting complaints about some of the imprecisions. You can also leave some of them on the comments section of this post.