Here're my approximations for surface area conservation

for a cube - edge lengths .8m for ever 2m the square would be (technically it's 2/sqrt(6))

for a cube without a top or bottom, edge length 1m for every 2m the square would be

for a cylinder or hemisphere, diameter 1.6 (cylinder has height .8m), for every 2m the square would be (technically it's 2/sqrt(2pi) and 1/sqrt(2pi))

for a circle, diameter 2.2m for every 2m the square would be (technically it's 2/(sqrt(pi))

for a sphere, diameter 1.1m for every 2m the square would be (technically it's 2/sqrt(4pi))

for a rectangle of height 2m, the length is half of the square of the ordinary side width (so 2m, 8m, 18m, 50m).