The size of the frame buffer is not defined by OpenGL, but by how the driver chose to implement the frame buffer. If it uses one frame buffer per window, then the frame buffer must be at least as big as the window to cover all pixels of the window. If it uses a global unified frame buffer (shared among all windows), it must be at least as large as the bounding rectangle around all windows to cover all pixels of all windows, or a fixed sized as large as the screen.

The viewport has nothing to do with the frame buffer size. It is only uses to specify the transformation from normalized device coordinates (the coordinate system after projection and perspective division) into window coordinates. It basically specifies what part of the window you're drawing to.

No, there is no real way to know. Other than for curiosity, there is no reason to know this, and if it is for curiosity, check for some product information about your graphics hardware.