I made a few refinements. This new shape is designed for vertical stacking up to whatever height is desired, with interlocking pieces to help maintain structural integrity and reduce vibrations. This is just a tweak to the previous design - the new arm isn't so strong, but it does eliminate that really nasty overhang. I have tested it out now (Thanks to some suitably British weather) and can verify that it does spin, and at a pretty decent speed too! Though without much torque.

This design works in two- or three-blade configurations. Unintuitive as it may seem, three blades isn't actually more powerful than two, except at very low wind speeds. Three blade have a different advantage: They run a smoother. Less noise, less vibration. A nice property of this design is that you can print a two-blade turbine out for testing, and if it shakes two much re-pupose the sections for a three-blade.

There's a catch to this design: It doesn't start easily. It's fine under no-load, but if you have any resistance to movement at all the it isn't going to start turning. You can hang it, or put it on a smooth rod, or build a pole with bearings to put it on.

You can print a turbine about 50% wider than the long diagonal of your build platform. That's as big as is practical for a printed turbine.

Note that you're never going to get high power output from a wind turbine unless it's really huge - but 3D printed models are fine for decorating the garden with and testing out designs. Sorry, but physics is against you: The power from a wind turbine increases with the square of the size.

Perhaps someone can find a way to make it useful - you can't really generate power off of it, but it could be used to drive a small music device that rings bells in sequence, or act as a bird scarer.

Now with demonstration video:
Mk7: https://www.youtube.com/watch?v=l2KNNG8fuXA
Mk10: https://www.youtube.com/watch?v=XcxPIb6TwHY

There are three parts: S (Straight), T (Twist) and a spacer (Also twist).

For two-blade use: Two straights and a spacer.

For three-blade use: You need two Ss and a T. Repeat the SST stacking as tall as you'd like.

You'll probably want two-blade though - it's quicker and cheaper to print, and very nearly as efficient. Note that the Mk6/7 parts are not compatible with the (far superior) Mk10 parts.

This thing has pretty pathetic torque, so you'll need to really get the drag down. Low-friction bearing. Get the surfaces smooth - a vapor or liquid solvent smooth is strongly advised. Once you've got the parts you need thread them on some rod, clamp nuts at bottom and top, and admire your garden ornament.