In order to determine proper gauge of welding cable you must first determine how long the distance you will need in cable, the entire length for both electrode side as well as work clamp side, and you also need to know the amperage you will be using. Once you determine this there are various references online and offline, most commonly in the owners manual to most welding machines, that will give you the proper size based on the amperage and total length.
there are a few different factors in determining the size of welding cable you use. Thickness of base metal, distance from machine to worksite, and amperage. I am a welder in the navy and sometimes we run our leads over far distances therefore we need a bigger welding lead to handle the increase in amperage to maintain the proper work temperature. There may be more reasons for using a bigger cable but I am not sure.