The -W switch in rtmpdump is included for the purpose of calculating the swf size and hash values for you.
All you really need do is include the -f switch giving details of the Flash version installed in your computer, and the -W switch giving the URL of the swf file specified in the webpage which serves the Flash file you want.
Example (for the Windows browser plugin, i.e. for Internet Explorer, where you have the Flash Player 9 release 260 plugin installed):
-f "WIN 9,0,260,0"
-W "http://www.bbc.co.uk/emp/9player.swf?revision=18269_21576"