Selecting a dunnage bag starts with measuring the actual void — the gap that needs to be filled between cargo, not the container's overall internal dimensions. A bag that's a close match to the void performs far better than one that's simply 'large enough'.
Bag construction also matters: single-ply bags are generally suited to lighter, low-pressure gaps, while multi-ply kraft paper or woven polypropylene bags are built for higher pressure ratings and heavier cargo loads. Reviewing the pressure rating alongside the cargo weight per bag avoids both under- and over-specifying.
For repeat shipments of similar cargo, standardizing on one or two bag sizes across a shipping lane simplifies inventory and training for the loading crew, and reduces the chance of the wrong bag being grabbed under time pressure.
When in doubt, sharing container photos, cargo weight, and void measurements with your dunnage bag supplier before ordering is the fastest way to confirm the right spec for a specific lane.