Gone are the days of the 1000px plus websites. Anything wider than 1054px is totally unsuitable and, in practice, the ideal width is much less.
Tablets like the iPad have a screen width of around 768px so the maximum fo this version of the site should be 800px since most users prefer to view web pages in Portrait mode. Any wider and the content will be squashed thus effecting visabilty and readability.
Smart phones such as the iPhone have a screen width of 320px in portrait mode so they require a page width of around that size. It's possible to go to a maximum of about 400px but, in this case, the font sizes need to be increased and user inputs increased from 30px square to 40px.