You have to set the resolution in the config. It has to match your desktop resolution. C:\Program Files (x86)\Steam\steamapps\common\IL 2 Sturmovik 1946 - Conf

[window]
width=1920
height=1080

Enter your own Resolution displayed in your gpu Driver.
Use OpenGl.

[window]
width=2560
height=1080
ColourBits=32
DepthBits=24
StencilBits=8
ChangeScreenRes=1
FullScreen=1
DrawIfNotFocused=0
EnableResize=0
EnableClose=1
SaveAspect=0
Use3Renders=0
WideScreenFoV=1
UIColor=0
UIDetail=0
UIBackground=rnd

[GLPROVIDER]
GL=Opengl32.dll

[GLPROVIDERS]
Open GL=Opengl32.dll
DirectX=dx8wrap.dll