from the Nats website: "All food items must be contained in single serving bags within a soft-sided container or cooler that does not exceed 16"x16"x8". Metal, plastic or glass containers of any kind (except for clear, factory-sealed or empty, plastic water bottles, no larger than one liter, juice boxes, insulin containers, and baby food) are prohibited at Nationals Park. Only one water bottle per person will be permitted."
so it looks like i could take my Yeti soft side carry cooler, it fits those measurements. that's awesome!