If it is size on an extension cord I think it may be around #4 or 6 conductor. As Jerry said I would have nothing less than #0 and preferably 2/0.
What I have found works well is welding cable ie type SO - IF you can use lugs to terminate them on battery. Otherwise stuck with premade with connections. Oversize on electrical is a good (cool) thing. Cool is the challenge on electrical keep it cool and all is OK.
You may be able to visit a welding shop or weld equip repair shop and get some scrap pieces of weld cable.
