How much solar sail and vane surface area would be needed to replace the boosts needed to keep the ISS in orbit? Could this be done at a higher orbit minimizing atmospheric drag?
Ignoring the ISS, the question is simply whether light pressure on the sail can counteract the drag on the sail from atmosphere. Light pressure near Earth is about $10 \mu Pa$ (with optimum geometry and no eclipse). So the question is what is the drag force per square meter of sail. We know that the ISS experiences about 0.2N of drag, and its area is dominated by the solar panels at around $2500 m^2$ suggesting a drag pressure in the vicinity of $80 \mu Pa$ at that altitude. So any sail will add more drag than boost.