The increase in diameter would be 0.01%. (12742km + 15km), increasing the surface area of the earth by about 1.2m square kilometers (to 1,512,469,109.7587 km2), about 0.25%
This really brings home just what an unimaginable volume of water we’re talking about! But it doesn’t sound like the atmosphere gets much stretched-it’s already a much deeper ocean on top of it all.
80% of the atmosphere’s mass is within 10km of sea level. So how much thinner would it be spread over a ~0.25% larger area?
I guess you’d be able to tell with a barometer, but I’d doubt you’d feel it, especially given the many other climate changes at hand.
