What is the minimum size of a modern site layout?

Question

Adaptive design

We want to make an adaptive layout.
I think that 90% of smartphones bought in the future will be 4'' or larger.
I can't figure out what the minimum layout size should be for 4'' size
iPhone 5 display is:
Resolution 1136 x 640 pixels, 326
ppi .responsinator.com
size: iPhone 5 portrait · width: 320px
Who to trust? What layout size should be 4'', 320 or 640 pixels?

Answer 1

iPhones up to 5s - 320 retina pixels wide. To understand how it works, check out this article: Designer's Guide to DPI

Answer 2

What to indicate in the T.Z. for the Designer so that the layout designer can then make a normal adaptive design for PC, Android and Iophones?
...completely confused with this dots per inch.