What would be the approach to build a web site that would detect whether to display the site on a standard desktop browser, an ipad browser or a mobile phone browser. I am making the assumption that the browsers in question would fully follow web standards.

What would be the sizes of the pages in question. I would assume I would need to replicate the site for each form factor, much like a multi-language site? Couldn't find much on this when I did a site search.
Thanks