If you cut a drywall outlet hole too big, don’t fret. It’s easier to fix than it seems. You even have a couple of different options.
Fix a too-big drywall outlet hole with tape and mud
I cut the hole for this light switch in my utility room way too big. I did a quick fix with some tape and mud. This needs a couple more applications of mud, sanded in between, to get it to look right.
You can fix a too-big drywall outlet hole with just some drywall mud and some tape. I use pre-mixed mud for this. If you mix up your own mud from powder, you want to mix it up nice and stiff. Closer to the consistency of mashed potatoes than to pancake batter.
Turn off the power to the outlet to be on the safe side. Then, using a 1-inch putty knife, glom some of that stiff drywall mud into the opening, starting from the edge and working your way down toward the outlet. The mud sticks pretty readily to itself and to the drywall, so the main thing to concern yourself with is getting a generous amount into the gap you need to fill. Then come back with a 2-inch knife and smooth it flush with the wall.
What you do next depends on how brave you are. You can apply tape now, and probably get a nicer result, but at the risk of messing it up. Or you can let it dry overnight. The next day, apply a bit of drywall tape over the wall for strength.
Apply one more coat of mud the next day over the tape and feather it out. You may need follow up by sanding it and applying additional coats of mud to get it to look good.
Patch a too-big outlet hole with more drywall
If you don’t want to mess with mud, you can patch the hole with more drywall. Cut a right-sized opening in a scrap of drywall a couple of inches bigger in all directions than your oversized hole. Then flip the piece over and trace an outline about an inch from the edge. Score along the lines with a utility knife. Then break along the line you scored and break off that inch or so of drywall from the edge. Now you should have a piece of drywall with about an inch of paper backing all around it.
Trace your patch over the hole with a pencil, then cut it out with a drywall saw. Insert your patch, then trace the paper backing around the area. Cut along your line with a utility knife, then peel away the paper. Now you can place your patch in the hole and secure it with carpenter’s wood glue. Glue both the gypsum wallboard and the paper.
Once the patch dries, fill in any small gaps that remain with a small quantity of drywall mud or spackle. Using this method, you can get a nearly invisible repair once you paint.