there are two ways i've found, one is using the Conf.ini file, the other is using an external program. the easiest is probably to use the program Il-2 Sturmovik Stab, which lets you adjust extra graphics options not normally in the game, such as resolutions for widescreens.
the other, using the conf.ini file, is mapped out pretty well on widescreengamingforum.com. here's the solution they have mapped out
Edit the conf.ini file in the root directory as follows:
[window]
width=(whatever your screens width res. is)
height=(whatever you screens height res. is)
ColourBits=32
DepthBits=24
StencilBits=8
ChangeScreenRes=0
FullScreen=1
DrawIfNotFocused=0
EnableResize=1
EnableClose=1
SaveAspect=0
Use3Renders=0
Set your height, width, color, and set the "SaveAspect" to 1. This last one ensures Hor+, according to some threads at the Ubi forums.
and that should take care of it for you
just don't mess with the il-2 config program again after or it'll default it back to 1024x768 or something and you'll have to repeat the process
cheers!