Start with the packed product
Measure the product in the state in which it will be packed. Include a backing card, outer wrap, box, handles or other features that need to fit. A soft item may change shape under pressure, so avoid relying on a single flattened measurement.
External size and usable space are different
Seams, drawstring channels, lining and a folded flap can reduce the space available inside. Ask for both the finished outer dimensions and the usable internal area. Mark the opening and the direction in which the item will be inserted.
Use a physical fit check
Place the real item in a sample, open and close it, and consider the packing process. If it is awkward to insert or strains the closure, revise the brief. Avoid treating an untested size formula as a final specification.
Record the agreed dimensions
Record width, height, depth where relevant, units and measurement points. Agree any manufacturing tolerance in the quotation or specification. Include a marked photo or drawing so different people measure the same way.
General sourcing guidance. Final specifications and commercial terms are confirmed for your project.