What Model and Generation do I have?
You can determine which iPhone 6S you have by checking the packaging. If you don't have the packaging, you'll be able to identify your iPhone by its size: the standard iPhone 6S has a 4.7in screen, while the iPhone 6S Plus has a 5.5in screen.
What Size is my iPhone?
Find out the size of your iPhone by selecting Main Menu > Settings > General > About, then select Capacity.The number shown here will be just short of either 16, 64 or 128GB.
If you are having problems identifying the generation or size of your iPhone then please refer to the original packaging or connect it to iTunes.