I have done cable before a few times but not a lot. My limited advice would be to start with the biggest piece you can handle with your equipment. That way you have some to cut off if there is a problem. If all goes well you will have some left for guards and bolsters. I do not remember Boss carrying cable lengths but I do remember seeing lengths of cable on ebay. The only other thing I will say is weld the ends up very well first. Trust me cable can quickly become a mess (at least when I do it).
Check with drag line crane companies. I used to get a lot free because if a line kinked then they had to replace the whole line for insurance purposes. I still have about 30 feet out in the shed for future projects/
I got some from Etsy a month or so ago from Official Yeti Forge, but haven't used it yet. They had multiple sizes and I got 5, 10" pieces of 1.5" diameter.
I also picked up 3 pieces of 1 1/4" and used 2 of the 3 so far from High Temp Tools. I do it by hand only, but I got a 1/4" thick, 7" or so long and 1" ish wide billet from about 2/3 of the 1 1/4" diameter cable piece.
I picked up an old used pipe vise and welded a 1" diameter bolt to it to use as a hardy tool to twist it tighter.
I soaked the cable in kerosene for a while and welded the ends and a work stick on to it. I also made up a stainless tray filled with kitty litter to protect the bottom of my forge.
Put billet in forge, get warm, either hit one end vertically on the anvil or untwist it a bit, put a bunch of borax inside and throw back in forge. Get hot to forge welding temp (yellow ish), take out and use the pipe vise to twist it tighter. brush off scale/borax and back into the forge as soon as it starts cooling down to orange before it gets to red heat. I repeated the twisting a few times until it condensed a lot and then started forging it out to the billet.
The cable compacts a lot, so I would definetly start with 1.5" diameter if possible and 10-12" pieces, depending on the knife size. I still have around a 4" long piece left over that I need to forge down more into a billet to see what I can make from it.