I'm making a webshop where customers can order photo printed articles such as photo on canvas, wood or metal. I already restricted the min width & height per design.
But the image can be scaled/zoomed so much that the quality would be very poor.
I've got an example of something great.
Gelato uses this in their designer: It measures the dpi of the image in the print area, so if you zoom or scale the DPI will be changed and if dpi is poor there is a warning.
This would be very clear to customers, and also for us sellers. because the client is being informed over how much he can scale a image without taking risk of poor quality.