There are a few things to consider when you go to longer blades.
1. Can they physically fit the heli (will they hit the tail rotor)? If they will, you need to extend the tail boom which can cause a balance problem.
2. They will slow down your cyclic response. The haevier blades have more inertia and as a result, gyroscopic force. Because of the greater ineretia they will improve the performance in an auto.
3. They will reduce your disc loading which will make the heli feel lighter in the air and increase auto performance.
4. Increased drag for the motor to push. This is negligable on a small change of 10mm or 20mm. As thin as the airfoil of most blades is, it is barely noticeable.
5. What blades are availible on the market. Obviously, if nobody makes the size you want that will limit your choices. I guess you could cut some oversized ones down but I don't like to alter something so critical to your flight.

The best way to find out which length to use is to experiment and see what you like. I have found that when you get to much blade you will notice a drop in performance and not much of a gain as far as disc loading and auto performance. Drag is an exponential factor so I'm sure this is the reason for that.

I hope this helps,
Malorie