your best thing to do is have a heating and ac contractor come out and tell them you would like them to run a load calculation and submit it to you with there price everyone gives free est. if they don't do not use them I run load calc. on every house we do and by state code you have to. There is no rule of thumb in this they can and will take your state lic.
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3,100
1,200
900 square feet
Rule of thumb is 1.5 tons per 1000 square feet of home.
to calulate your btu needs,you can use this link: http://hearth.com/calc/btucalc.html