Frontal area would increase by 6.6% (using the old π*r(adius)^2 formula for the area of a circle, and your 20cm figure).
Wetted area (total fuselage skin area - i.e. friction drag) would increase by the added circumference of the fuselage - 2π(radius). About 100 m^2 additional skin surface to enclose the wider cabin, or 3%
Now, the fuselage drag is only a
part of the total drag (parasite drag from other shapes (wings, nacelles), induced drag from lift). So the increase in total drag might be 3-4%. Less at lower speeds (climbs), but significant in high-speed cruise.
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That, of course, leaves aside how much more airlines would have to pay Boeing, per aircraft, to completely revise the jigs and tooling to accomodate differently-sized airframe parts. Including, for example, control runs, floor panels, or a new ceiling (unless you'd accept a 20cm gap down the middle overhead between the carry-on bins
). Do you use the same wings, and increase the span by 20cm? Or shorten the wings 10cm each to maintain the current span? etc. etc.
Thought experiment - you want to increase the size of your house by 20cm in every dimension. How much would it cost to tear out the walls and roof and rebuild them, offset by 20cm?