There's a useful index for various cases at:
Beam Deflection Calculator - Engineers Edge
You also need to know material properties for the wood - which you can look up online.
Then it's a case of choosing a trial balloon - pick a beam size, calculate stress and deflection, and see if you're within the material limits (including factor of safety).
If that's undersize, or significantly oversize, try another beam size and repeat the job.
Don't forget to include the dead weight of the beam.