Parcels can be rejected by service points due to their size. To prevent this you can enable the "service point check dimensions" option on your integration.
Currently this option is only available for WooCommerce integrations using the SendCloud plugin.
By entering the correct dimensions of products in the admin of WooCommerce (and enabling this option), the service point picker will filter out carriers that can not handle products of that given size. Note that there is no advance packing logic behind this, we simply check if all given products are within the maximum dimension of the carrier.
Below is an example of how to setup maximum dimensions in your WooCommerce product settings.