In rotationally symmetric lens design, there are rule-of-thumb boundaries on field-of-view and aperture for well-known design forms that provide valuable information to the designer prior to starting a design. In the design space of unobscured three-mirror imagers, freeform optics have been shown to provide a significant benefit over conventional surface shapes, but the degree to which they improve the performance for any given combination of field-of-view, entrance pupil diameter, and F-number remains unknown. Thus, designers of these systems are not afforded any pre-design information to inform their specification decisions. Here, we designed over 200 systems to establish a first-of-its-kind roadmap of specification ranges over which an unobscured three-mirror imager using freeform surfaces can achieve diffraction-limited performance in the visible spectrum. The scalability of the findings to the infrared regions of the spectrum is also addressed.