Bleed and Safety Margin

Add "printable design" option.

This will enable a predetermined border inside the product editor to help guide the user. It will only be in editor and only for enabled designs in size settings.

or users can enable it to be seen like rulers

There would need to be two

.125in for bleed (37.5px) 

and another .125 (which is really .25 in inside// 75px) for safety.

Or it could be a predefined image the Admin uploads for custom sizes. So in View options you can select the image and its opacity or again to be used as a guide button in editor like ruler