The dimesions of the lumber you use for the roof is dependant on the pitch (rise/run) of your roof. for say a 6/12 roof that has an 8' span 2x6 should be plenty. If you have never cut rafters before spend $15.00 on a Stanley adj rafter guide. It comes with an excellent pocket book on how to and you can use it as a guide for dimensions of your lumber. Another source is your local building code. Good Luck!