Please try this.
Remove the first underscore from the window_size_ratio= line like this

windowsize_ratio=1.0

The way it's generally written by most panel design program makes it an invalid command.

Depending on the size of the main panel BMP the size_mm=1280 would cover the screen. Make this the actual size you want the pop-up window. Change it to something like size_mm=150,250 This defines the actual ratio of the window. Make the gauge the same size as the window & position it at the top left corner of the window. Now it's active you can easily adjust the relative size of the window + gauge by changing the windowsize_ratio= value.

You might need to delete the window_size= line. This will possibly conflict with the previous one now it's activated. You should end up with something like this.

[Window04]
size_mm=150,250
windowsize_ratio=1.0

VISIBLE=0
ident=RADAR_PANEL
window_pos= 0.440, 0.560

gauge00=WRADAR3,0,0,150,250


My tute should give you the idea. http://www.simviation.com/lair/AddGPS.htm
This is based on the old FS98-style panels. The position of the pop-up windows is not so accurate using the position= line but I find it works fine for most purposes. Please take a backup copy of Panel.cfg before trying it. I'm not sure this method will work using the later window_pos= line without the window_size=  line.