From the Olympics website:

3. Bring the smallest bag possible: If you have a bag smaller than a bread box (15 cm x 15 cm x 30 cm or 6 in x 6 in x 12 in), or no bag at all, you can use the express lane to enter the venue. Spectators carrying larger bags will follow the standard lane. Lanes will be clearly marked.

IS IT BIGGER THAN A BREAD BOX?

*dies quietly*