For just the one.. min five feet as red bellied pirahnas can grow to be a foot long. Should really be kept in groups as random said.. but if you do go for just one you could possibly keep it with cichlids.
Mod's note: Each and every member of this forum is entitled to ask questions and explore possibilities, without having others comment on or question his/her circumstances or intentions.
In the future, please show some respect for each other.
bettas - here are a few links with lots of info for you to check out if you are serious about piranhas. Your profile does not say where you are from, but you need to be aware that piranhas are illegal in some areas (I know as a fact they are illegal in California).