The heavier the vane, the less easily the blades will be turned out of the wind. A very light vane can cause a turbine to spin repeatedly around its yaw mount (swivel mount), as it can start to oscillate in a strong wind.
For one of my generators, 450mm back from the pole, a weight of 3kg just about balances the weight of the generator core plus the blades. So whatever weight and vane pole length is chosen, its best to have the total weight times the distance from the mount pole to the centre of gravity of the vane about equal to .45 * 3 = 1.35kg m
Your generator might have different magnets, rotor plate thickness etc., so it is best to check this for your setup. To measure this, mount the generator on a workbench (I do this using a hole I drilled to size through the top of the bench). You will see it lean forwards (away from the vane). Use a scale (fish scale works well) and measure how hard you need to pull down on the vane mount to get the generator to stop leaning forwards, and be approximately vertical. Multiply this weight by the distance back from the mount point, and that is the figure for your setup.