Your report will indicate which part/s of your product data are incorrect. Data will fail where it does not comply with the specifications of the National Product Catalogue "Data Dictionary". Compare the stated reason for your data verification failure with the relevant section of the guide.
Dimensions must be entered in millimetres with product sides, tops etc. defined according to the GDSN Package Measurement Rules (attached document).
Your aim should be to achieve the greatest possible accuracy. However tolerances for dimensions and weight are defined in the GDSN Package Measurement Rules (attached document).