i think its hilarious that there is two separate conversations here, one about discouraging piranhas in the aquarium and one about disposing of bodies. However, neither really answer his question... save for 1077's 125. The size of your tank will be fully dependent on the size of your fish. Assuming you allow them to all grow to adulthood and maintain a population of 6m you would have 72 inches of fish. However, the fish need extra space to mimic their environment so one should assume it would need double the inch per gallon leaving you at 144. Go with a 150 with a filter that's designed fora 300 gallon tank. The Red Belly Piranha
I will have to chime in on the discouragement of these fish in home aquariums. As the article also notes, this is NOT a fish to be kept just because its entertaining when they feed. One thing you really need to consider is your experience with taking care of fish. This is a fish for advanced aquarium hobbyists. If you aren't willing to dedicate the time and money to this project, you should consider learning to care for easier, more passive fish. Also as many sources noted, piranhas are not as aggressive in the aquarium environment. Lastly, note that the fish do like meat, you will be putting your hands in that tank to clean it, do you want to take that risk?
As per disposing of bodies, one could easily chop up the victims and feed bits to the piranha each day and freeze the rest... Yes morbid, but not wholly ineffective. However, i have heard that fish such as piranhas do not enjoy human flesh as much since we have an excess of iron in our blood.
All in all, if you do decide to get these fish and dedicate your time and finances to it, good luck. They are beautiful fish and im sure successful care will yield wonderful rewards.