Bag sizes at King Baudouin Stadium

My girlfriend is going to a concert at King Baudouin Stadium and she wonders how strict are the size rules for bags to bring in. Their website says anything larger than an A4 will not be permitted.

Does anyone have info on how strictly that is enforced? Could she get in with a bag that exceeds the limit by 4-5 cm? Or does it depend on who’s checking you in?

She says hi and thanks in advance for any intel!