Hi
The easiest way would be to put a notice on your /customize/ page beneath the customizer.
Any size image is allowed for upload, they will be sized down to a practical size for print when uploaded through the app.
The file size is limited to a 20MB upload.
If you would like a feature added inside the customizer app, please use our Get a Quote form and describe the exact feature for our dev team to give an estimate.
Thanks,
– Sean