I have wiped my iPhone 6s and want to sell it. How do I find out the amount of memory it has from the iccd number

If you bought it through them do they not have records?

Or do you have your original email receipt or order?

I am not in USA and not used to these restrictions but AFAIK you just stick the SIM in and set it up. Then afterwards it's Settings, General, Transfer or reset iphone, Erase all content and settings.

If they locked it out it will not set up.

Nope, as long as you followed these instructions -> How to unlock your iPhone for use with a different carrier - Apple Support and got the confirmation message it was unlocked after erasing it, it will not lock to any carrier again, ever.

Your best way is to set it up agsin with your appleID, take a screenshot of the Settings, General, About screen and airdrop or text it to another device. Then wipe it again.

In the Hello screen if you swipe it left there is a lot of info but I can't remember if storage is on there.

If you unlocked it via AT&T it is now permanently unlocked. It will not lock to any carrier again.