With pitchprint you can make designs online or upload your own. For online designs you can set proportions and dimensions but it's not available for designs created by customers. I suggest beside design to be used in online editor set list of parameters to check customers designs. In example when customer uploads business card system checks DPI, size, color model, file type. If everything matches user can upload and order, otherwise error is shown to indicate what customer has to correct.
Also would be great if system would put transparent PNG with bleed lines and safe zone on top of preview of customer's design to assure that design prepared correctly and nothing will be cut off.