If you need a new Photoshop document the same size as the one you have open, there are two ways I know of to create one without actually checking the document size dialog box. First, you can select all and copy the background layer of the open document, then Command + N for a new document. Photoshop will automatically set the size to whatever you copied to the clipboard, which in this case was the entire background layer of the open document. The second way requires a trip to a menu, but is just as easy. When you have a document already open, hit Command + N, while the new document dialog is open, click the Window Menu in the menu bar and select the open document’s name.