After you select an area, perhaps by using the 4 pixel/degree or 1.6 pixel/degree map, the inline maps in the Mars Atlas (here, for example) are 16 pixels per degree (1/16 degree, or about 3.7km, per pixel). They are available at 256 pixels per degree (about 230 meters/pixel) from CDROM, though some setup is required to display those. The 16-pixel/degree scale was chosen as a compromise between detail on the one hand and disk space (and downloading speed) on the other. (In theory the map tiles could be read directly from the CDROM and scaled to any desired size on the fly, but in this Mars Atlas they are all cached on disk at 16 pixels/degree.)
While 230 meters/pixel is the highest resolution at which maps covering all of Mars have been prepared (as far as we know), the raw Viking Orbiter images from which all the maps derive are often 80 meters/pixel, and sometimes better. Like the original map tiles, some setup is required to display them, but 1/8 scale copies of them can be displayed inline (here, for example).
See examples of different scales.
Kanef@Ptolemy.ARC.NASA.gov