To begin the research i have looked into the top handheld devices out at the moment, extracting the dimensions from specifications and tabulating the results, allowing for us to decide what the most appropriate size of our product should be.
In the table above i have included two different types of handheld devices, 3 are PDA's and the other two are handheld gaming devices. The reason i have included the second two is that they are intended to be held with two hands and so give an indication of boundaries where a device becomes too large to use with only one hand. We must also keep in mind that we are dealing with elderly people who are going to have decreasing mobility, strength aswell as eyesight so keeping the screen as large as possible while the whole device as light as possible will certainly be high in the specifications.
