There is no default size for working with websites in Photoshop, it all depends on the usage. Today most designers will work with a variety of sizes that respond to the device/browser size. A good indication of some of the more common sizes can be found in frameworks such as bootstrap, and have breakpoints along the lines of:
Large screen: 1400px
Tablet portrait: 1024px
Smartphones portrait: 320px
These are just examples, but personally if I was designing for a desktop view I would work on a 1400px basis. Any reason why you are designing for flash by the way? It is no longer the standard way to create a website as there is minimum support.
For more on common screensizes: http://www.w3schools.com/browsers/browsers_display.asp For more on common responsive grids: http://coding.smashingmagazine.com/2011/01/12/guidelines-for-responsive-web-design/