Measure the merch before the bag
The bag is the container, not the starting point. If you collect larger badges, thick charms, sleeves, or plushies, take rough dimensions of the merch first and compare them against the listing.
A bag can have a generous body but a tighter display frame. That is why sizing should always be read as both total bag size and usable display window size — they are not the same number, and listings that only publish total bag dimensions without window dimensions require extra scrutiny.
The items that most often catch buyers off guard on size: acrylic standees (taller than they look in photos, need at least 10–12 cm of vertical window space), large enamel pins (some full-size character pins reach 8–10 cm), and plushies (a palm-sized plush that looks manageable in a product photo often needs 15+ cm of window depth to fit without being compressed).
Take three measurements before you search: the height and width of your largest single item, the rough width of the full display you want to build, and the depth you need for any items that are not flat. Those three numbers map cleanly to window opening height, window width, and interior depth in a product listing.