A lot depends on how long they need to be and what size rods you intended to weld with. If relatively short on 2.5mm and 3.25mm (5m or less) then 25mm2 would be ok.
If you want to get further away and or run 4mm electrodes then you’re looking at 35mm2 cables.
Duty cycle also has a bearing. The lead gets hot at the ends (dinz connectors and or the clamps) The heavier the leads are they take away the heat from the cable ends (terminations) and put less heat into the dinz / socket or the return clamp. The cooler things run the happier
Oxfords welders tend to live in one place in a workshop, generally close to the main fuse box, so the leads are as long as required. Mine one had 30ft min (35sq mm) with Dinze connectors, so extension cables could be added if required, but my electrode holder had about 15ft of super flexible 25sq mm cable fitted to it which kept the holder light-ish and a joy to use.