Knowing the accurate dimensions (often abbreviated to DIMs) of your job will help providers give you the best quote. But If you don't know exactly how big or heavy your items are, then simply tick the box 'Unsure of Weight or Dimensions'. However you will still be required to enter an estimate into each of the dimension and weight fields to submit your listing.

If you wish to edit your listing to include more accurate dimensions at a later date you can either EDIT your listing (which will decline all quotes) or send a MESSAGE to each provider to clarify small changes.

More details on edit and message features here: Can I Edit my Listing?