I have a similar issue, I am chasing on modifying the default "frame" size of the source element for the stereotyped image created with a shapescript...
Okay, got that working (redrew image to occupy 100X100 in X/Y in shapescript and adjusted shape proportions accordingly) then added...
fixedAspectRatio = "true"; defsize(100,100);
When loaded via MDG, dragged onto diagram Frame Same size as Image... then used corner to shrink as needed.... changed the font, etc. since it is a package that I stereotype, once I adjusted it... I can't just copy and paste.... have to repeat those steps each time

Could add some automation code later....
David