1px = 10km
or
1px = 100kmĀ²
at the equator.
to clarify:
the vertical side of the pixel will always be 10 km
the horizontal side of the pixel at the equator is 10km
at the poles it is 0km
where d is the distance of the pixel from the equator in pixels;
the horizontal side of a pixel is sqrt(1024^2-d^2)/1024 kilometers long
the area of a pixel at the equator is 100km^2
the area of a pixel at the poles is 0km^2