Your bag must be 16” x 16” x 8” or smaller. That’s the hard limit. Think of it like airline carry-on rules, but with more hot dogs and fewer tantrums. Anything bigger? You’re out of luck, pal.
Honestly, that size is generous for a day at the ballpark. You can fit a hoodie, a water bottle, and a bag of peanuts in there without breaking a sweat. But—and this is a big but—they will measure it if it looks suspicious. Don’t try to squeeze a duffel bag in there. It won’t end well.