On the HiRISE map, you can display the footprints of prior observations. What is the size in kilometers of these footprints?
HiRISE uses "pushbroom" imaging so the length of the footprint is variable depending on the length of "Time Delay and Integration" (TDI) used in a given observation.
However, the latest MRO HiRISE instrument SPICE kernel (ik) from NAIF (v12) suggests:
At the nominal 300 km MRO orbital altitude the instrument can acquire image swaths of approximately 6 kilometers cross-orbit and 20 kilometers along-orbit.