Any particular reason why you require a file with dimensions beyond 6000px?
Why does that matter?
Just curious - single web page designs can easily accommodate a length which exceeds the 6000px value of Fireworks. And I believe it should not be set much higher than about 10000 pixels, or there may be issues.
Ordinarily you change the value in
If changing the MaxDocSizeInPixels in that file does not work, try searching your entire file system for other preferences.txt files. I am unsure about Windows 8 - should be identical to Windows 7.
I've done that, but it looks as if the Creative Cloud version of FW overwrites the Preferences.txt file when launched. So when I change my local Preferences.txt file to say 8000px, this local file is overwritten by creative cloud with a 6000px version. I think that's what's happening anyway...