I have seen both some of the largest and smallest genomes among animals (well, I have seen stain bound to their DNA, at least). The largest report remains that of the marbled African lungfish, Protopterus aethiopicus, at a gigantic 132Gb (about 40 times more than humans). Some authors argue that this is an overestimate, but regardless they have huge genomes that are undoubtedly much, much larger than those of any mammal.
Until recently, the smallest animal genome size was reported to occur in some root-knot nematodes of the genus Meloidogyne (~ 30Mb) or perhaps in the placozoan Trichoplax adhaerens (~ 40Mb). I tended to have doubts about the nematode estimates because they were derived using older methodology. However, my colleague Serge Morand and his co-authors now report an even smaller genome in a plant-parasitic nematode based on estimates using modern flow cytometry techniques. In particular, the genome size of Pratylenchus coffeae is estimated at about 19Mb, making it the smallest so far found in a metazoan.
Leroy, S, S. Bouamer, S. Morand, and M. Fargette. 2007. Genome size of plant-parasitic nematodes. Nematology 9: 449-450.
Image from http://nematode.unl.edu/pracoff7.jpg