According to this article, an A-10 or C-130 could operate from a 4,000 foot runway if it has a hard surface (e.g. steel matting). You might need some more if the field altitude is high or if you're carrying heavy loads from the field. If possible, I'd allow for another thousand feet to provide some safety margin.

Such an airfield could also be a base for planes like the AV-8B Harrier II and even F-15Es and F-16s. It'd also be useful for all sorts of helicopters and AC-130s. The hard part is logistics. You'd need a lot of support to bring in the necessary fuel, spare parts, and munitions. An airfield in Northern Alliance terroritory would be useful, especially if it's outside of Taliban artillery range.