I believe as long as its smaller than 16" by 16" by 8", its allowed. One bag per person though.
"Bags To provide for a safe environment at Kauffman Stadium and expedite entry into the stadium the Royals have outlined the only bags approved for entry into the stadium. (1) Clear bag no larger than 12” x 12” x 6” with all items inside easily visible. One-gallon clear zip-seal bag (Ziploc® bag or similar). Small clutch bags that do not exceed 9" x 5", with or without a handle or strap, may be carried into the stadium along with one of the clear bag options. An exception may be made for medical reason or manufactured diaper bags that accompany infants and young children after proper inspection" From the website
I had a medium size cross over bag. I took out my phone/battery pack/ wallet for the lady to check [not much else in the bag ] worked out.
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