tom7railway
A permanent magnet DC motor can be connected to a source of mechanical energy like a wind or water turbine, and it will generate electricity.
I have a small electric scooter motor hitched up to a pushbike, and it can generate over 100 watts, if the cyclist is fit. It's bigger than a fist though. If you want a more powerful, yet small generator you will have to spend a lot more money.
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I hope you mean just the head for that size thing - if you are referring to the blades, you can't put enough air through to get enough energy to make any worth while amount of electricity.
A better request would be "I need to get 100 watts of electricity from the smallest possible wind turbine. Where can I find one and how small?"
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If you google "small wind turbine, you will see dozens of small wind turbines. Best of luck finding the right one. Here is just one of many links.
roger
look for bicycle generator.
bird b
EBAY.