Install Load Bearing
the size of the beam is based upon the loads it must support, the type of beam being used, and the span of the beam. This is not a simple matter as a mistake made can be costly. Structural design is not something a person should come to an internet forum to obtain.
To properly assess the path of gravity loads (as well as lateral loads) a person needs to visit the structure and observe for themselves the existing construction. I would recommend hiring a local professional engineer to come to your home and evaluate your needs and prepare construction drawings.
Typically performing structural work requires a building permit and meeting the local building codes. A call to your building department will determine that. An engineer can prepare the necessary drawings and specifications so that you do the job correctly the first time .....
"You get what you pay for, and sometimes free costs more!"