Free State
Daiku was on the right track. However, M= wL^2/8, for a uniformly loaded beam , where w = load per lineal inch (or foot) and L = length of beam (in.). To calculate w, multiply the total load (in pounds per square inch) by the spacing of the beams (in inches) or if the beam is used singly, use the supported width of floor area. [Note: the bending moment formula depends upon the type of loading condition. Here it is given for a uniformly loaded beam only.]
Once M (the bending moment) is known, use the material property Fb (psi) to calculate the section property S (in.^3). S is given as the (width x depth^2)/6.
This should give you the dimensions of the piece required. I hope this helps.
Mark Gillis