For most ArcIMS image results, the default size is 400 by 300 pixels. The exception is an image resulting from a GET_LAYOUT request. The size of the resulting image of a GET_LAYOUT request will vary.
A GET_LAYOUT request is used in a custom client to access the layout of an ArcMap document. The default image size of a GET_LAYOUT request is based on the page size set in the ArcMap document (in inches) and the default dpi of 96.
For example, if your ArcMap document page size is 34 by 44 inches the default image generated will be 3264 by 4224 pixels.