A quick google search on the Red Belly Piranha shows they grow to at least 12", reported to 18" in some cases. They need roughly 15-30 gallons per fish--so you'd be looking at roughly 5 fish for a 150 gallon. They can also live up to a decade, so if you get them I hope you're prepared for a long term committment.
RO water seems to be highly recommended as well.
If you're thinking of getting piranha, please at least learn to SPELL their name. It'll make doing your research alot easier.