In the picture you need the pixel size and 1 room real size
here is a ex.. room that is 125px by 125 px, the ex.. actual sizes is 500 qu by 500 qu. the scale is 125px/500 qu
or 0.25 px per qu
or 500 qu/125 px
4 qu per 1 px
the map is then the same scale across the whole picture.
Note to other --> Please check my process/math
Take Care --
EDIT2 --> I believe the forum and AUM have many ways you can search for and I also believe there is a shader based solution...