That is a question for the beam manufacturer. They have tables for this. Go to the lumber yard and speak to the sales man. He can size the beam for you from the manufacturer charts.
You do realize there is a lot more to this than just installing a beam. You need proper support at both ends that transfers load to proper load bearing points.
You'll need to provide the lumberyard's engineered beam tech quite a bit of dimensional information regarding your house so they can size the beam appropriate to the load. They'll need all sorts of dimensions to figure what the load is. If you have your house plans that'll help immensely.
It definitely isn't a service that anyone can accurately provide for you online.
Yes, he'll need to know if the bearing wall carries floor, roof, ceiling joists and all that stuff. You said it is a bearing wall so I assume that you've crawled up into the attic and checked it out. Be on the ready with how big the spans are in your roof if it carries roof load for the guy selling you the beam. He'll be alot cheaper then getting the beam sized by an engineer. Like the guys said, check that the load can be transfered to a suitable spot downstairs if you have a downstairs.