SIM

iPhone 7 and iPhone 13 use the same size sim cards. However, you shouldn't use the sim card from an iPhone 7 in an iPhone 13 anyway. If you want to use a physical sim card, you'll want to get a new one from your carrier which has been provisioned for 5G service. You don't actually have to use a physical sim card in an iPhone 13. You can contact your cellular provider and as long as they support eSim, they can set up your phone number on your new phone without a sim card at all.

As I said, the 7 sim will fit in your new phone, but you really need to either get a new sim card or have your carrier activate the eSim in your phone.