It depends on the form, 2 baisc forms of koshi nage are head under or hips under (as taught in Iwama) with head under there is a wider position of the feet, here is an example of demonstrating where to align the body.
http://www.youtube.com/watch?v=M-9cNnd3bFU
for the hips under its usually a smaller space between the feet as in this version
http://www.youtube.com/watch?v=ODzpi_rLM7k ( although this teacher is very tall so his feet are naturally wider anyway for his balance)