Keeping object size when changing window size

I need some larger render frames without the object getting larger with it. Is there a way to do this? or do I need to mess with the lens until it matches.

Move the camera away.

Fiddle with the Lens setting in the camera edit buttons (F9)

I gave a more exact answer to this exact question in: https://blenderartists.org/forum/viewtopic.php?p=230031

I thought that might be the only way.