first off if its only 1 fish in that tank ... I would recomend that you put in 2 new ones so that way they can spread teh aggresion out and not pick on each other 3 RBP's in a 55G will do fine...try to get new ones as close to they same size as possible
I added five 2" reds to my large red (5,5" by then) in june last year, and they're all alive and kicking! The big one is about 7", the small ones 4-5" give or take.
I'd go for at least 3,5-4", but smaller ones may work out as well.
And like the others said, get at least 2 new fish, and the more the better. Two reds will kick each other's asses until one will die (all they can do is attack each other). If you have more than two, they can spread the agression. And the more you'll get, they more at ease they'll be (when you have a tank that's large enough...).