personally a 30 will be fine for some time. only problem is socializing this fish. it can be very hard to re-introduce isolated fish to a social environment, later down the road. i am using this as a reference of possible breeding in the future. but its totally up to you, and he/she would be fine for some time in a 30 IMO.
There's really no maximum size for any fish, I would say.. what fish wouldn't like more room? But a minimum for a flowerhorn, full grown, I'd say 75+, but a 55 should do.