If your fish is smaller in size, then smaller tanks may be used but in the end you still need a bigger tank so we always suggest a 3 feet tank as a beginner tank.
Flower Horns living in bigger tanks usually grow bigger in size as compared to those living in smaller tanks under the same feeding condition.
Some people have asked us whether decorations should be included in the tank. We feel that bare tanks are better because Flower Horns eat a lot and bare tanks are easier to clean up as compared to tanks fully decorated. You may of course have background poster and lighting to brighten the tank and your fish.