The setup is designed with 14mm blade roots. Therein lies the problem. It's really designed for m-blades which have 14mm roots as stock, so it wants to run 1050mm blades.
However Volker designed a different gearset to allow increased headspeed and you can fit OF 875 mm blades which do come with a 14mm root.
If you do go this way, tell Volker when you order, that you want to run Fiberglass blades and he will supply the larger pinion and smaller intermediate gear to raise the headspeed.
If you want smaller than 875's, you will compromise the lifting capability of the heli, but I would guess the smallest you could go would be 800's without another increase in headpseed. With 800's I am guessing it would lift itself off the ground and thats about all.