The base fits a DC motor with 20mm in diameter but its dimensions can be adjusted on the openscad script.
This is a work in progress. The plastic is so lightweight that it doesn't counteract the pull of the motor and I'm hoping it'll eventually work as a mini propeller (test video: http://www.vimeo.com/8199151). But for now it really is just a cooling/exhaust fan which can be held still by either bolting it down or using the 9V battery version - the weight of the battery is enough to keep it from moving, specially if placed on a rubbery surface.
I'm sure this warning isn't necessary for most of you, but I'll feel better if I mention it anyway: make sure the shaft is tight and the fan is mounted snugly on the motor. If it's even a little bit loose the blades' piece might fly off and injure someone. Test it facing away from you and others.