Built around the packed set
Cosmetic packaging usually holds a mix of shapes: bottles, tubes, compacts and applicators in one pouch. The internal size comes from the packed set, not from the largest single item — a pouch sized from the biggest bottle alone generally will not close over the whole kit. Decide early whether it lies flat or needs a gusset to stand, because a gusset is a different pattern rather than a larger version of a flat pouch.
How the zip performs in use
A zip contains contents properly and adds structure, which is why it is the closure specified wherever small items would otherwise escape. It is also the component most exposed to wear: the tape, the slider and the pull are three separate quality decisions, and a cheap zip undermines an otherwise good pouch faster than any other single part.
Run the zip repeatedly on a packed sample. Smoothness under load, and whether the slider wanders off the tape at the ends, are not visible in a photograph.
The fabric
Fabric weight is the specification that matters most here and it is set from what the pouch has to carry or protect, not chosen by appearance. Send the heaviest expected contents with your inquiry so weight and seam construction are specified together.
