PHOTOSHOP REMEMBERS THE LAST SIZE YOU INPUT

Although there's no way to set a default size for your New document dialog (outside of creating an action to do it for you), there's a pretty cool little-known tip that lets you open a new document using the last settings you manually input. Just press Option-Command-N (PC: Alt-Control-N) and the New document dialog will appear using the last settings you input (rather than using the clipboard data to determine the new document size).