I use two cake mixes with the cake extender recipe and it seems to give me the right amount. I am guessing that the cake extender gives me about 2 cups.
When I use DH I add one box of pudding and an extra egg to each mix. This consitently gives me 6 cups of batter from each mix. So if you are only useing the mix it will give you a little less but It has been so long since I measured that I don't remember how much it is.
Below is a link to Wilton's charts for 2" and 3" pans. I use these as a guidline when filling all of my pans (wilton and magic line)
http://www.wilton.com/cake/cakeprep/baking/times/index.cfm
HTH
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