The image memory limit is based on the desired maximum size of an output image and its resolution. By default, the image memory limit is 4 MB, which allows output image dimensions of 1024 x 1024 pixels (that is, 1048576 pixels). Any request for an image exceeding this size will result in a "Requested Image too big and cannot be created" error.
For ArcMap services, consider the page size of the layout when determining the image memory limit.
The file size of an ArcMap Service layout request in a vector format (ai, emf, eps or pdf) is not restricted by the image memory limit.