You can find your iPhone model number by looking at the small text on the back of the iPhone.
There should be something that says "Model AXXXX". Match that to the list below to find out which iPhone model you own.
You can find the size of your iPhone (16GB, 64GB, etc.) by opening the Settings app in iOS and navigating to General > About > Capacity. The capacity section will be slightly less than what you actually have. For instance, a 64GB iPhone will probably show the capacity as around 56GB.