If the GSX window is too large for you when you open it, there are 2 ways to solve it:
1. The first is temporary. Open the GSX window and resize using the arrows on the edge. Now, each time you open it, the window will open to that size and position until you restart P3D at which time it will revert to its original large size.
OR for a more permanent solution
2. The size of the GSX window is controlled by a "SIMCONNECT_WINDOW" entry in the missionpanels.cfg file located in the "\gauges" folder.
a. Open missionpanels.cfg with notepad and look for the following entry:
[Window12]
size_mm = 375,250
position = 4 ; Middle Of Screen
visible = 0
ident = SIMCONNECT_WINDOW
scaleform_file = MenuWindow ; Use the generic menu window scaleform content
scaleform_instance_name = SimConnectWindow ; Provide a unique instance name
b. after the size_mm line, add the following line where X.XX (horizontal) and Y.YY (vertical) are a percentage of the main P3D window from 0.00 (0%) to 0.99 (99%)
window_size = X.XX, Y.YY
I have a 32" widescreen and "window_size = 0.40, 0.50" works nicely for me. You may need to experiment to get the ideal setting for your monitor.
CAUTION: DO NOT SET X AND Y SO LOW THAT SOME ITEMS FAIL TO APPEAR.
c. With this line inserted, the "size_mm" line is ignored, and since it is a percentage of the main P3D window size, the GSX window will resize as you increase or decrease the size of the main window.