In 2006 I was teaching web design to students at the Australian National University in Canberra. These were computer science students at Australia's leading university and so found web coding a bit too easy. To make it harder I had them redesign web pages for mobile devices. The difficulty here is not the coding, but deciding what information to leave out. Rather than target a specific phone, I had them design for a credit card sized screen. This, I suggested, was as large as a smart phone was ever going to get, as anything larger was not going to fit in the average pocket. It was also, in landscape, about the width of a newspaper column.
At arms length a credit card (or business card) is also the same effective size as the typical flat screen TV in a lounge room, or projection screen in a lecture theater. The average reader holds a book or smart phone 400 mm away and a credit card sized screen is 85.7 mm wide. So doing the maths, a 1 m wide wall screen can be viewed from about 4 m and appear the same size as the smart phone display, and a 4 m wide screen from 20m.
Today the typical size for a smart phone screen is about the size of a credit card (there are larger phones, but they are bulky). When preparing presentations I suggest designing for this size screen.
With some smart phones now having micro-HDMI out sockets it is possible to present on a big screen from a pocket size device. This may result in improved readability of presentations..