I have had frogs breed in a small garden pond of about 100 gallons. I have some quite large Leopard Frog tadpoles in this pond right now, who look like they're thinking about turning into frogs. I plan to build one or two larger frog ponds in the next year or two.

One should include diverse plant habitat around the pond, lots of rocks and logs, and not have fish in the pond (they may eat eggs or small tadpoles). Apparently it's best not to have a pump because frogs prefer still water to breed in.