The fan only has to be large enough to move the air around and keep a hot spot from forming at the top of the incubator. It depends upon the size of your incubator as to what size flexwatt you use. A larger incubator will require larger and more Flexwatt, a smaller one will require less. You don't say what the size of your incubator is going to be, so it is hard to give specific advise. Although, as long as you can maintain the proper temperature and have the air circulated, it should be OK.
Edit: I use 11" flexwatt in my incubator, but could probably get by with smaller. The larger Flexwatt seems to hold a more stable temperature for me.
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