Yep, what he said.
Even 5/16" - G43 is probably all you will need with a CK unless your really pullin' extraordinary heavy stuff with it. Get the dealer to weld you on some bucket grab hooks.
Strength wise, 1/4" will likely hold all you can pull and then some, but I like to go bigger just for extra safety. I also go with gr. 70 because you can get more strength with a smaller chain. If you plan to trailer it, look at the requirement for tie-down and buy enough grade 70 in the strength you need to put 4 corner ties on the tractor. Most states require separate ties for attachments including the FEL, backhoe, and even the back hoe bucket might require separate tie off. I usually use nylon straps for the attachments but chains for the tractor.
My advice is unless you need a box blade immediately, forego getting a new one. Look for a used one and save about 50% of the cost. It seems almost everyone, myself included, buys a box blade. I haven't used mine in years although I did use it a bit to level my shop location prior building. I also bought a landscape rake that I thought would be useful for grading my interior road, and it does work, but I can back drag it with my
B26 FEL in less time than it takes to hook up the rake so it has become a chicken perch and they seem to like it as a bathroom location. Same with my box blade. Lots of folks use them for grading gravel roads but since I have such short road, I don't need it for that.
I haven't used my disk harrow in a couple years now since I got my King Kutter II tiller. I use it instead of the disk for garden prep. I even used it on a pasture fire once to cut a fire brake at over 8 MPH and it did a great job.