No compatible tank mates, puffers need to be kept in species tank only, they will nip, stress and often kill tankmates. Exception to the rule would be a bumblebee goby (brackish fish)
Tank size and whether or not it's brackish would depend on what puffer you're talking about. The smallest puffer would be the dwarf puffer, which is completely freshwater, but still requires a min. tank of 5G and 5G per puffer.
Do you happen to know which puffer it was? If not, can you describe it to us please? Or perhaps have a look at pictures of puffers online and see if you recognise the puffer in question.