That depends on your box’s configuration. In any case, a parcel can be no bigger than 68 × 65 × 51 cm. That’s both the maximum size of our XXL compartment and the maximum dimensions for parcels that bpost picks up.
Please also note that your parcel must be placed at the bottom of the compartment; otherwise, it will not be detected.
Also make sure that your parcel does not weigh over 30 kg.