Yeah flowerhorns can get to around a foot or so in length (around 12-13 inches) they usually get aggressive when the hit maturity at around 6 inch or so but mine is only around 2 1/2 inches and already got a bad attitude.
They need tanks of 55 gallons minimum but you would have no chance keeping any other fish with it. I have mine in a 5 foot 450 litre (125 gallon) and even in that it would be hard to find a tank mate. I think he will be fine with my bichir though as it is already 8-9 inches and can hit twice that size eventually. Also I'm hoping because they have been together since the flowerhorn was small they will be ok together.
Iv got an 8 foot 800 litre (180UK/210US gal) coming soon as soon as I buy 2 fx5 filters so my flowerhorn and bichir may be moved into that. Not sure yet