You need to know your building width and the distance you are spanning. Call your local lumberyard with those numbers and they will size one for you according to the manufacturers specs. It can get tricky because there are snow loads, wind loads, seismic conditions, codes, etc. If there are any special conditions too, you need to take them into consideration as well. You might need an engineer or architect.
You just don't guess what you need. Something you guess on might look good initially but in the middle of the winter when you have 4 feet of snow on the roof it might fail. Or you could go overboard and put something in that's way overkill and spend extra green that you don't have to and end up performing extra work to accomodate your overkill.
As long as you don't have any special conditions your local lumberyard will be able to look it up and tell you what will work.