Mine Is Bigger than Yours
Macromedia decided a long time ago that 550 x 400 was the perfect default size for the Stage because 550 x 400 is the largest a Flash movie can be if it needs to fill a browser window on a monitor with 800 x 600 resolution. Although the logic makes sense, when was the last time you created a movie with dimensions of 550 x 400? For me, it was about a million years ago. Furthermore, because many Flash projects require more than one .swf file, it might be more convenient if new Flash documents could be created with custom settings.
Open the Document Properties (Modify > Document) dialog box and change the Stage dimensions to something you normally use or will use a lot for a particular project. If you need to, change the background color and frame rate as well. Then click Make Default, as shown in Figure 1. Every time you create a new Flash document, the Stage will use these settings. When you move on to a new project, change the default settings again. It's a one-click operation.
Figure 1 Make your document properties the default for all new Flash documents.