I have a lounge floor, 16 X 12 feet wide, which has 5" X 2" joists at
11" centres along the width (12 feet). I have removed a toilet and
pantry underneath, the walls of which helped to support the floor in
question. The floor is ok but a bit bouncy, since it should really
have 7" joists. I intend to put a 16 foot RSJ underneath at 7 feet
along the width. Any help in calculating the size of the RSJ? How
thin can get away with, bearing in mind that anything would improve
the situation? Hope all this makes sense! Any help greatly
appreciated!
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You should have put the steel in first before removing the wall, as the joist/floor will have deflected and sagged. You now wont have a level ceiling. Is this a concern?

It can be pushed back up with some acrows, and the RSJ packed up at the
bearing.

dg