Fitting an object to an artboard in Adobe Illustrator
I can see how to fit an artboard to the art therein (Object -> Artboards -> Fit to selected art), but how do I do the opposite, i.e. how do I automatically resize a selected object to the artboard bounds?
You can not "do the opposite".
There is no internal option to "fit artwork to artboard". What you are seeking does not exists.
You may be able to write or find a custom script for this though.
If you press Shift-o then Enter It will show you the width and height of the selected Artbord.
You can manually enter these dimensions to your object by first
Selecting the object, going to the top toolbar and clicking transform
You can then center it using the Align to Artboard settings.