Anything over 4" should be fine. Only problem is that the arowana is going to grow much faster than the Flagtail, so the larger you can get the better.
I was thinking of getting a 4-5" one to go with my 8" red aro. I dont want it to look too big next to my aro. Isnt this a good size to go with? Also I am looking for one with a red top fin. Anyone seen them around toronto lately?
Check with Oliver at Below Water. He can get them in for a reasonably great price, and are spectacularly colored.
Most of the ones you are going to see are the red finned variant. If you can find the yellow finned consider yourself lucky. As they age, normally the fins get more and more intense red in them.