Earlier post says 75 gallon.
With a child that may get attached you may want to stay away from the powered blue tang. They will outgrow that tank. There are a couple smaller tangs like the Kole eye (yellow eye) or tomini tang or that would suit that size for the most part. If you did want the "dory" then you can buy small but if she gets too large/aggressive you may need to find her a new home.
Other fish from that movie include a royal gramma, a yellow tang,
Damsels are pretty and cheap. Some people used to put them in the water as sacrificial fish without properly cycling. If they are on your wish list then they could be first as I mentioned before... cheap.
My favorite inhabitants are my hawkfish, one spot foxface rabbitfish. And chocolate tang (mimic lemon peel tang).
Just some suggestions
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