How are you viewing the image? It could be that the DPI info being written to the image is wrong, and if the app you’re viewing with (Photoshop for instance) uses that info to correct for pixel aspect ratio or something similar, that’s the problem.
If you load the rendered image back into particleIllusion as a background layer in a new project, you should be able to see that its dimensions are fine.
Alan.
http://www.wondertouch.com