|MadSci Network: General Biology|
A variation on your question was answered in this previous Mad Scientist response to a similar question:
The largest animal is the blue whale.
The smallest animal...well, it depends on what is classified as an animal; this is what is discussed in the previous answer, along with various answers according the definition/ classification of what is an animal.
Try the links in the MadSci Library for more information on General Biology.