scale ortho images in autocad

I need to extract ortho images of elevations that I can use in Autocad. My idea is to extract high res images that I can convert to png files (smaller) then draw over them in Autocad. How do I scale the image correctly in Autocad? What would be the best settings to use in merge? I've tried a few but there are many options and I think it will take quite a while to go through all of them and find the optimal settings for each.