Glulam?
Microlam?
LVL?
Fir?
Spruce?
Pine?
Box beam

Lot's of kinds of wood in this world - all with different peculiarities.

i am planning to use a standard douglas fir beam(solid) that is available from Home Depot.

Engineering advice from an internet site like this with no other information about the design of the house is worth what you pay for it.

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

I brought a coop and the kitchen floor is sloping! What can be causing this? My neighbor has a ceramic floor applied over thinset concrete.Our kitchens are back to back.Could this be causing the sloping or a poor design when building the coop be the problem?