Each account has a configuration setting called "Reply Image Size Limit." It specifies the maximum reply image width and height that can be returned to the client. The server returns the "illegal image size" message if a request would cause a reply image whose width or height is larger than this setting.
To adjust the MaxPix, log in to Scene7 Publishing System (SPS), and choose Setup > Application Setup > Publish Setup > Image Server > Reply Image Size Limit. Increase this setting to a value you think is sufficient, and save it. Then run a publish and the image. The current limit is 4000 x 4000 pixels.
In addition, the Adobe Scene7 servers currently don't serve up images that are larger than 16 million pixels.