A 150 gallon tank would probably be big enough to do a "Nemo" tank. This is completely my own opinion, and if anyone strongly disagrees, please say so.
BUT, make sure that if you are determined to do a "Nemo" tank, that you put the fish into you tank in this order (again, if anyone disagrees, please correct me):
-striped damsel
-percula clowns
-royal gramma
-starfish and cleaner shrimp
-"Gill" - he is a Moorish Idol in the movie, but please don't buy one, they don't survive well in tanks - get a henochious butterfly instead. They look almost identical (definitely to a kid) and they have a much better survival rate, though they do better in schooling groups of three.
-blue hippo tang
-porcupine puffer
-yellow tang - the most aggressive and should be the last fish introduced into tank
It's a lot of fish, but if done over the course of a year and the tank is maintained extremely well with excellent filtration, it is possible.