Set the finished size
Share the product width, depth and height. Leave enough room for the product to fit and for the bag to be carried comfortably. The finished dimensions should be confirmed against a physical sample.
Record the width, depth and height of the largest product or typical combination of products. Include any inner box or protective wrapping in those measurements.
The bag needs enough space to pack the product without forcing it against the sides. Its opening, gusset and handle clearance should also make it comfortable to use.
Share an approximate size to begin the discussion, then check the proposed width, gusset and height with the actual product and a physical sample.