There is no such thing. These days one will use different layouts for different devices. A tablet has other requirements from a mobile phone and those are different again from smartTVs, Desktop Computers, others. Unless you design to exact spec for one of those, it is best practice to use fluid, adaptive layouts. But really, it's more a web thing than anything to do with Photoshop and you have some reading up to do. For Desktop use with fixed layouts you should consider the smallest possible screen it may be viewed on and that still are netbook screens which have about 1366x768 pixels. For anything else - see above.
Mylenium