I don't think the AoE will increase exponentially with Haki. Continent is even far above what the Gura Gura no Mi can do with a single move. I don't even think one swing could destroy Wano, since Wano is a land you can travel for many days.
I think Dressrosa is a good benchmark. Cutting through the whole country including its edges, which is a mountain range surrounding the country. Dressrosa is pretty huge. A mountain like Pica when zoomed out looked small, and it took Zoro some effort with CoA to cut through Pica.
You could argue Mihawk's slash goes beyond the edges too. But continental is too much for anyone in the One Piece World.
I think Dressrosa is a good benchmark. Cutting through the whole country including its edges, which is a mountain range surrounding the country. Dressrosa is pretty huge. A mountain like Pica when zoomed out looked small, and it took Zoro some effort with CoA to cut through Pica.
You could argue Mihawk's slash goes beyond the edges too. But continental is too much for anyone in the One Piece World.
He already did that way back in East Blue when this was unheard of
East Blue Mihawk
MF Mihawk with Minor power creep
EoS Full power Mihawk with Final Power creep
He will cut the chunk of earth's surface