how do I determine if my box is too large or too heavy

No millimetre standard, but a rule on empty space

There is no exact minimum size requirement for a box in the regulation. Instead, a ratio applies: the empty space in packaging must not exceed a set maximum percentage of the total internal volume. This means that the box itself is not measured, but the air inside it — a box that is too large for a small product is thus the problem, not a "box that is too large" in itself.

This particularly affects shipping boxes, because the difference between product content and box format can be largest there. Filling material such as paper padding or air pillows does not count as useful content; it concerns the space that the product and any protective packaging actually occupy in relation to the external dimensions of the box. For those who are uncertain whether a shipping box falls under this separately or is counted together with the product packaging, the distinction described in the difference between sales packaging and transport packaging is relevant, because the empty space rule specifically targets the packaging that is supplied to the end user.

The legal basis: Article 24 of Regulation (EU) 2025/40

This rule stems from Article 24 of Regulation (EU) 2025/40, which sets out the empty space ratio for packaging supplied to the end user. Because it is a regulation, this requirement applies directly and equally in every Member State; there is no need for national transposition and no country where a different standard could apply. What does vary depending on the situation is whether packaging, as plastic packaging, must also comply with the recycled content requirements of Article 7 — that is a separate obligation, apart from the shape or fill level of the box. Those who must keep documentation on this aspect can find it combined in the answer to the question what documentation must be kept with packaging and for how long, as substantiation of the empty space ratio is part of the broader packaging file.