I'm no expert but as I understand it the larger the rotor disk the lighter the disk loading and the more responsive the heli gets. Also the gyro action of the blades is more so the stability is enhanced as well. This is true of the flybar also. Most Trex owners run the 325s although the Align blades are actually shorter than advertised because they measure then overall instead of from the bolt hole like they should. This means that the Align 325s are actually 315s and so forth. If you buy aftermarket blades they will probably be longer.
I am running the Align 325 Pro wood blades and like them but I have some CF blades on order to see how they work.