They are accurate to what you upload. The excepted 150 dpi files and what ever measurement you have at 150 dpi is the size they print.
If you upload an 300 dpi image it's prints twice the size then the original file because it double the size compare to the 150 dpi file.
Rochelle skirt is flocked black onto a rainbow fabric. O_O
There a product called Flockfolie that one Merida cosplayer used to add flocked detailed pattern to.
Another option is to layer velveteen over the mirror fabric.