The short answer to what size wire to use is the largest you can afford.
http://www.powerstream.com/Wire_Size.htm
This link goes to a nice chart of wire size and the recomended amp capacity. If you try to save money by getting wire that is too small you will waste generated electricity in the wire and potentially create a fire hazard.
Batteries in a RE system typically charge at a slow rate for a long period of time and discharge at a high rate for a short period of time. Therefore smaller wire can be used for the charging circuit (wind gen to battery) but larger wire must be used for the output (battery to inverter).
If you think 100 Watts at 12 volts is the most your generator will produce, the answer is that it would be moving 8.3 amps which calls for 12 guage wire. I've had good luck with low voltage outdoor landscape wire from the local home improvement store. If you attach a 300 watt inverter to the battery, it will pull 25 amps which calls for 8 guage wire. I would go 6 or 4 guage to handle surges and upgrades. This is common battery cable size, available at the local auto parts store.
Make sure your wire connections are solid. No wrapping around the terminal and hope it stays. Crimp lugs are great, mechanical lugs (screw to hold the cable) work well for their flexability. I suggest spending a few extra bucks at the auto parts store on getting cables that have the correct connectors already attached.
I strongly recomend reading prior postings on this site. A search on the word "battery" will give you plenty to read. At first it's overwhelming but if you keep at it the info starts sinking in and it all makes sence.
By the way, in this project, either the RE bug will bite you or it won't. If it dosen't, you will learn a lot, have some fun, and what you put together will end up in the back of the garage. If it does bite you, you will learn a lot, have some fun, and what you put together will end up in the back of the garage as you improve and upgrade your system! Therefore what's important is that you learn and have fun.
-Henry