Well, I used a 6' box blade with my ~3000# Jinma 284. I'm not familiar with the weight of the CK20. Depends on what you will use it for and how fast you want to do it. I struggled with heavy clay in wet conditions, but had no problem with anything else.
As the BB was considerably wider than the Jinma, I was able to grade to the side without driving over it first. That kept the tractor level and therefore the BB too. At least the BB should cover your tractor width.
Now I use the BB on my DK45, and it's almost toyish, but I can still bury it enough to make the DK work. Hope that helps.