Flower horn grow extremely fast, and can get 12 inches the first year, most will be slightly smaller than that, but they get big.
They are super aggressive, while there is always exceptions to the rules the general idea is that you cant keep them with any other fish. Even super large tanks. I had two, and have a six foot tank, i tried to put one in there and it fought with every other fish in the tank, they were african cichlids and quite aggressive themselves.
I love the look of a single large FH in a show tank, they are so colorful and have beautiful fins and personality. I would house a single flowerhorn in a 75 gallon tank, as a juvie it can be in a slightly smaller tank, like a 55 or 70 tall, but as the fish grows it should go into a 75. Remember get only one or they will kill eachother, it usually does not matter if they are male/male or male/female, they will often try to kill eachother. I have heard of some limeted success keeping them with blood parrots, mostly people who were trying to breed them, the idea is the blood parrot can breed with it and if it dies its a inexpensive fish to replace. I find that idea, kind of repulsive, i dont believe in throw away fish, so i kept mine by themselves. I had one in a 72 bowfront and one in a 75 gallon, they liked it and grew from 3 inches to six in about 6 to 8 weeks. like i said super fast growth rates.