for the grid dsm (5cm spacing is really dense so it can do really big files). i don't know what is the area covered by your dtm, but with high density mapping with a total station it should be more something like 5m spacing.
To generate contour lines with pix4d you can use the "additionnal output" option in the dsm/orthomosaic output menu. and choose the autocad dxf option.
To write the elevation every 5m in the autocad drawing is a little bit…"