Square surface around geodetic coordinates

Hi,

I would like to define a square by its center point in geodetic (or also geocentric) coordinates and its edge length and plot it in geodetic/geocentric coordinates.

So to say:

The centerpoint is 40.0°N and 30.0°W and the square has outer dimensions of 100 km x 100 km.

How do I project this cartesian square on the spherical surface of earth and plot it?

Thanks a lot!

Cheers!