Identify my IPhone

Go ahead and set up the phone again very simply and get the info that you need from the General category. You can skip the Apple ID setup and other security / passcode items for this very simple temporary setup.

When you have the info that you need, go ahead the perform another Factory Reset on the phone.

I wish I remembered more.. but I thought I played with that at one point and was able to get some of that info while my iPhone was not setup.. From the Hello screen, is there a little i in a circle for 'info'. Are you able to tap on that and see anything?

if that's all it gives ya, and you don't have the box the phone came in.. then you would probably need to do what Bob Timmons is suggesting.

Only other thing I can think to try.. is if you have the SN from that info screen, go to

Apple Trade In - Apple

and select Smartphone under Select your device to get a trade-in estimate.

After plugging in the serial number, if I recall correctly is provides some details and asks if this is the correct info.. maybe that says the GB size