There is no prescription one for all. Available space is the space that is at least 1ft away from any superstructures on the roof, sometimes 1.5ft. So if you have 9ft from airconditioner to front bedroom vent, you in fact have 6 ft available for panels. If any tilt is planned, it becomes yet more complicated. You want enough clearance between the panels to avoid shading. With big titled panels, say over 200W, it is preferable that tapping screws in legs hit the roof rafters, because large panel will cause a lot of wind-induced stress.
