Measure the length and width of the duct opening. This looks like a 10 in X 6 in register would fit. You can find this Home Depot. The register can be a little smaller and does not have to be an exact fit. Your register sits on the floor so will have to subtract 1.5 - 2 inches from the height measurement.

Thank you. Do 10 x 6 would not fit because it sits on the floor?

Going by the dimension of the 10 X 6 register I found at Home Depot, the minimum height opening you would need is 8 3/4 inches. It looks like it would fit, but it is hard to tell from the photo.

Mitchell, just take the register cover that you have removed with you to Home Depot or Lowes. Match it, face to face and back to back, with the ones they have in stock there. They will have replacements for you

Just be aware that having a damper in the register will restrict airflow and may create a lot of noise.

That appears to be the return air duct, that brings the air from the room back to the furnace. They typically do not have levers to adjust. I would only have adjustable vents on the air coming into the room.

Are you trying to block all airflow in that room, for what purpose?

Thanks. The air is pouring out of it when heating or ac is on so I don’t think it is return. I want to divert sone of that air flow to other rooms upstairs, as bathroom is a very small room.

I had a vent that didn't want to close up once. I just got some tinfoil and put on the inside of vent, it worked!!

It will work until you find the correct size.

"I want to divert some of that air flow to other rooms upstairs, as bathroom is a very small room."

This is better achieved by "choking" off the duct to that particular supply, not closer than 12' from the register.