From what I have gathered from the odd picture of them they have to be huge to actually have enough area to power them, they also should fly high and be a wing. The US did a project like this and it was something in the region of 10m across!
I have just started to think about this. My goal is to build a solar powered UAV that can cary a smartphone and Arduino Micro. My initial search of existing designs suggested that most of them were overgrown park flyers. My brother has been working on some UAV software for Android that seems like it has a lot of potential. With what he has already done, we can stream video and telemetry of a cellular data connection. It seems like some really cool things could be done by combining a cellular connection with the ability to stay up for hours at a time (potentially all day).
My main concern is cost and the thing I really need help with is solar cell selection.