I have googled unsuccessfully. I have data about voltage drops etc.
But what is acceptable?

Is there anywhere that I can find what are of wire I need, given my
expected current and length of wire?

I ask because (Hi. I'm troy McClure...You may recognise me from...) I
am setting up a welding traveller based on a Cobramatic push-pull
welder. I need to extend the wired from the power source to the
feeder, and also the erath electrode wire. I also may need to install
a solid wire from the feeder to the gun, rather than the water-fed one
that's there at present. While the water fed is superior, they cost a
small fortune, unless I can cobble one up, which may or may not be
worth the trouble.

I have asked a couple of welding shops, and had vastly (in?)different
replies, both between thenselves and from the wire that is already on
the welder.

Data

Welding current 220A max MIG
Distance from welder to feeder 5 metres 916 feet)
Distance from feeder to gun = 8 metres (25 feet)

Existing wire intended to go from welder to feeder
length = 2 metres (6.5 feet)
area = 70mm^2 !!!!!!

I am sure the welding shops were talking a lot less than this. I do
knwo that the same feeder is designed to provide power to a 400 Amp
gun, if required. So it's all swings and roundabouts.

Any help appreciated.