I have a question. My fiance lives with his mom now. His brother used to live, but he joined the coast guerd and he moved to Alaska. Ok. his mom works fulltime, and so does his brother.

I will live with him and his mom. So acutally three people in our family, but his mom does not depend on him. On the form of 134, he put only my name. Should we count that our household size is 2 people and calculate minimum income based on 2 households? which is14000X125%?
please let me know.

Yes count only you two! If you count more dependents then the income level must be adjusted up for each person. It goes up. So unless you make serious money I would leave it at 2. That way you should be able to beat the poverty level for 2 people by 125%. If you can't beat the income level then a co-sponsor can be added but you must also claim them as another dependent and anyone else they support. Be careful with this unless you need the co-sponsor. Take care and good luck.

Yes, the household size will be two. He must earn $17,500 it's the 125 poverty line for two people.
http://www.uscis.gov/files/form/I-864P.pdf
$14,000 is for sponsors on active duty in the U.S. Armed Forces who are petitioning for
their spouse or child.

Good luck