If you click & let go of the mouse button in the same position, select that room and change focus with that room at the centre of the map.
If you click, drag, then let go, move the map focus accordingly, but don't select a room.
If you click the right button on the map, zoom out to a 'whole level' scaled map or perhaps 6x8, 9x12 (whatever fits) view. From there, if you click on a room, it zooms in with that room as focus, but does not select it. If you right click again, zoom back to where you were before.
For any of the above, if you're playing, the next 'move' key would switch back to the current room at default zoom.
Finally, introduce higher screen resolutions (with accompanying higher resolution art
) Then you'll have extra space for the map in the first place.
I wouldn't personally like to see scroll bars. IMO, they would look messy, unless perhaps they can be hidden when not in use.
I'm not sure about switching away from the game entirely to view the full map. I've a personal preference for getting as much done as possible without switching screens (unless the interface becomes over-complicated) It might be nice to have a full map option though, through a hotkey or perhaps a left+right click on the map.