Maps are “infinite,” not limited to pre-calculated volume (the “Everywhere You Click - There Is Electron Density” (EYC-TIED) paradigm) symmetry-related electron density is generated automatically. Maps are easily re-contoured. Simply use the scroll wheel on you mouse to alter the contour level (or -/+ on the keyboard). Maps follow the molecule. As you recentre or move about the crystal, the map quickly follows. If your computer is not up to re-contouring all the maps for every frame, then use Draw -> Dragged Map... to turn off this feature.
Unfortunately, there is a bug in map-reading. If the map is not a bona-fide CCP4 map 1, then coot will crash. Sorry. A fix is in the works but “it's complicated”. That's why maps are limited to the extension ".ext" and ".map", to make it less likely a non-CCP4 map is read.