The club is not obliged to act in advance - fewer than a third of clubs in League Two have any sort of policy in relation to bags, and there is no law currently obligation them to act.
The law currently going through Royal Assent makes no mention of bag size - it merely says that security measures and searches in relation to bags should be robust, thorough, and enforced.
We choose to limit it to bags under A4 size - we don't have to and won't have to.