I am working with a designer who wants to cram a lot of content into the mobile version of his website and who has provided a design at 750px physical width rather than at a more average device width.
As he designs in Photoshop, I want to be able to go back to him and say "show me a version which is 320px wide and has fonts no smaller than X pixels" in order to try and emphasise quite how little space we actually have on an average phone, and why his template needs to be simpler.
The smallest fonts will be only used for small print t&cs and so just have to be legible rather than optimised for easy reading. The likely font is going to be Open Sans.
What font size is the smallest possible in this case?