They get up to a foot long, hope the link helps.

Here is a link that might be useful: Red Piranha profile

Probably a red pacu.

Here is a link that might be useful: Red Pacu description

Try putting your finger into the tank and see if it will destroy your finger, so you will know if its a deadly Piranha or a peaceful Pacu. Dropping a beta into the tank does not answer your question, since large fishes consider betas a glorious snack.