A group of 5 on up. Also someone mentioned that they will kill off the others and that necessarily is not the case. Many times they do not do anything to the others unless they are spawning. Also they will show that they have paired off by their behavior usually even before spawning.
What size is up to you.....do you care if they breed when they are smaller or would you prefer to keep them separate until they are the size you WANT them to breed at? Main thing to remember is bigger females produce more eggs, and larger male are more sought by females due to stronger genetics