Do I need to know the exact bag size?
No. Product dimensions and intended use are enough to begin discussing a suitable finished size. The construction should be confirmed with a sample.
No. You can begin with the dimensions of the product or box that the bag needs to hold. Describe how many items usually go inside and how the customer will carry them.
The proposed bag width, gusset and height need enough room for packing and handling. The opening, base and handle position also affect whether the size feels right.
Use a physical sample with the actual product to confirm the final size before approving production.