It is possible to resize the GPS window during any flight session. However, it will resume to its default value once FSX is terminated.
To make the change permanent:
- Open the panel.cfg of the aircraft to be changed
- Locate [Window02] section
- Decrease the value of window_size for smaller window (0.4 or 0.35 represents 80% or 70% of the original size), or
- Increase the value of window_size for larger window