ORIGINAL: Shadow99
The stock blades are 235mm long, I've used Venom's Night Ranger 3D night-blades (245mm) with good performance as well. Anything longer than 245mm may cause serious issues with the tail rotor (main blade/tail blade interference), unless you extend the tail.
Shadow
Actually, the stock blades are 10 inches and are listed as 254mm. The Carbon blades are listed as 9.9 inches on Towerhobbies and are shown as 252 mm. I've got plenty of room between the main and the v3 size tail blade, so you could go with bigger blades just becareful of overheating the motor. An inch longer or shorter will have little change to the flight characteristics. The question is where to find blades of this size?