weight a wood beam will carry
Lot's of kinds of wood in this world - all with different peculiarities.
A fifteen foot sapn is likely to require a man-made LVL or glulam. You give no details about what it will support. Roof only?
or just taking the bounce out of some floor joists midspan with no actaul loading?
header for three stories above?
Complete plans are necessary for complete answer but a generic rule of themb would call for this beam in fir to be a 4 x 18 or 4 x 16