How do I find iPhone storage capacity using serial numbers after a reset?

The serial number is only used for identification purposes and cannot be used to find the storage or any other internal information. But if you have reset the iPhone you can assume the only storage being used on the device is just the iOS and System Data.

Or if you have the original box or proof of purchase receipt, that should include the storage information.

You will have to set up the phone again to get into the settings and view the storage capacity settings. You don't have to do a full setup or transfer data from another phone or backup, just a quick plain setup.

Then, you can erase the phone again to sell it or give it away.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you don't want to set up the phone again.....and you are asking about the iPhone 16 Pro in your profile.....then you will have to assume that you had the basic model with storage capacity at 128 GB.