how do I find the battery size on I phone 7 plus phone in settings

There's nothing in settings that will give a battery size/capacity. The only tool that I know that does that right now is coconutBattery for Macs. It has the current charge, full charge, and design capacity - all in mAh.

https://www.coconut-flavour.com/coconutbattery/

You don't. Apple has never officially published that detailed information for any iPhone model.