I got my new gas range installed and want to remove the 50 amp breaker for the electric range and replace it with a dual pole 20 amp one for a workshop in the basement. I was going to cap off the electric range wire in the panel, but when I opened it up, I see (I think) 6 gauge aluminum (looks like aluminum and it says 6AL-3 on the cable). So, I'm not sure I can get an appropriate wire nut for that. So, instead, I'm thinking of just removing the cable and use that knockout for the new 12 gauge wire. I'm pretty sure that cable clamp is too big for the 12 gauge, and my 3/8 trade size cable clamps won't fit inside that large knockout. I think the diameter of the knockout is 1 1/8 inches, judging by the knockouts around that one. I see there are reducing washers for things like this...

So, my question is, what size reducing washer do I need to get? Obviously the outside diameter must be bigger than the knockout hole. However, I don't know how they are "sized" (physical, or trade size like that 3/8 inch trade size clamp for 1/2 inch knockout) & I don't know how much larger the outside diameter of a reducing washer needs to be over the knockout hole. Do I get a 1 inch one, a 1 1/4 inch, or what? I'm assuming the size for the reduced hole is 1/2 inch (the physical diameter of the reduced knockout--for example, a 1 1/4 x 1/2 reducing washer). The local big-box stores have a small selection, so I may need to order them, which means I may not have the luxury of being able to just measure them with a ruler before purchase.