Is it possible to observe more area in azimuth than physically traveled in stripmap SAR?
It's so that the cross-range (along-track) resolution is consistent for all targets in the scene. There is no reason why you could not use the flight path between [-5,5] to form the same size image. However, in that case, the targets on the edge of the scene will have been illuminated for a smaller angle than the targets in the middle of the scene, which will given them a smaller region of support in the frequency domain and, hence, a coarser resolution.

When choosing the first and last sensor locations that are used to form the image, it's common to select the first point to be that where the beam first enters the imaged scene and the last point to be when it leaves it. Picture the footprint of the beam on the ground as the sensor flies by; it's usually an oval or cone or something like that. At -20 is where the edge of the oval (or whatever shape it is) begins to intersect the imaged scene. At +20 is where it stops intersecting it.

I should add that, in general, the size of the imaged scene is limited only by the PRF of the sensor.