Hi, this is a matter of pre-sale ;
Something I do not understand what is the management of the size of the « stage » relative to the actual size of the product to customize.
I take the example of an iPhone 4 case to illustrate my point :
The real size in mm is 62 mm x 120 mm to 300 dpi . Making it in pixels : 738 x 1420 px.
So I understood that we can scale the " stage " with these dimensions. But the page will be high on the screen ;
Now imagine that I want to propose customizing posters or photographs printing. A typical size for a poster , for example : 500 mm x 700 mm x 300 dpi. Making it with photoshop pixels : 8268 x 11811 pixels !!! The page of the site is going to be huge !!!

Is is expected to have a dissociation between the appearance of the product on the screen and its true size for printing?

Thank you and sorry for my english.

Yes! The stage size should be the scaled down version of the print ready dimension. 

And. these 2 features are a must:

1. The stage size should not be the actual size.
   
2. The print ready file should only contain the customer's design. In the example above by kameleon Premier, the poster background should not be embedded in the print ready file.

So how does this work then?

How do you get the file uploaded by the customer in print ready size and also appear on the preview in right scale?