As Bruce said, the size of a beam depends on many things: the span, the load (i.e., supporting a roof, supporting a 2nd floor, snow load, etc.). You should check with the local building inspector or an architect to be sure of the right size. But, Iíll tell you, Iíve never seen a 30í wood beam with no columns under it. You could do it with a steel I-Beam for sure but I donít think you can do it with wood. Even a large glu-lam or micro-lam beam(s) is probably not going to do it. Maybe someone else has an idea.