Was putting a circuit in an electrical panel, and tried to get the
smallest knock-out out, but of course the largest (2"?) came out. So now
what do I do? Go get a 2" connector for a single wire or is there a device
that can close off this huge hole?


*You can buy a knockout seal to close the 2" knockout and then use another
knockout in the panel. You could also buy a pair of reducing washers
2"x1/2" to use that opening for your connector.

Are you sure that is a 2" (Trade size) opening? Usually 2" concentric
knockouts only go down to 1" or sometimes 3/4", but every manufacturer is
different.