well, it depends on how much pitch you want, which way the peak will go, or do you want a shed style roof.
I will explain a shed style roof using your specified measurements. First, you need to picture the end of your roof as a triangle. In algebra 1, we are all taught that to find the length of an unknown side of a right triangle, you use the pythagoreon theorum.
A squared plus B squared equals C squared.
SO.... your building is 6 feet wide. add 1 foot for an eave, making that 7 feet.
Next, let's say you make the front wall 2 feet taller than the back wall. this makes the short side of your roof triangle 2 feet.
Now for the numbers magic.......
2 squared is 4
7 squared is 49.
49 + 4 = 53
this is "C" squared. to find the length you need, find the square root of 53, which is 7.3 feet, or 7' 4".
8 foot lumber will do nicely.
If you want a regular type ranch style peak roof. then you need to do this.
half of 6 is 3. half of 7 is 3 and a half. ( this figures for an eave.)
2 foot rise. back to Mr. Pythagerus. Or however the heck you spell it.
3.5 squared is 12.25 feet
2 squared is 4
12.25 + 4 = 16.25 feet
the square root of 16.25 is just a smidge over 4 feet.
SO..... an eight foot piece of lumber will do if you cut the eave back to an inch or two.
Hope this helps.....