Does the table have any leaves that can be removed to change the size of the table?

Your rug should extend approx 3 ft around the perimeter of the table.

Maybe you require a custom-sized rug.

I like the table because it is so big. And I have considered a custom rug but I still would like some opinions on the sideboard/credenza being on the rug too.

I would personally forgo the rug. I've never been a big fan of rugs under dining tables. In design it looks pretty, but they aren't practical.  All the spills and chairs constantly moving on them make them hard to keep looking nice.  

How much room is there to the credenza presently, ie: a 9x12 custom with a border added. If it won’t make a difference for the table’s needs, maybe skip a rug.

It’s not ideal to have a rug basically going up to the walls on either side as room will look odd.