I see, I was under the impression it would be much larger. I double checked and turns out I was confusing it with Plex which is the space hog (currently storing 10GB metadata although that is stored on my NAS). In that case 1MB/series should be fine. I’ve got ~220 series and the OS drive is a 30GB SSD with ~1GB free space.
I did have a play around with file junctions and created a hard link between c:\ProgramData\MediaCover => d:\Sonarr\MediaCover (mklink /d) but it duplicated the data which isn’t what I’m after.
Are you sure its duplicating it? The files in C:\ProgramData\NzbDrone\MediaCover should be pointers to those in D:\Sonarr\MediaCover, but shouldn’t take any disk space.