I'd like to play Darklands in a 1600x1200 window. I can set the windowed resolution to that (either by using the GoG configuration utility, or by hand in the dosboxDarklands.conf file, and when I run the program the initial screen (the one with white text on the grey background, that starts with "Darklands(TM) - Version 483.07") displays properly in a 1600x1200 window.

However, immediately after that it snaps to a smaller, 980x770 window as the opening animation sequence plays, and from that point on the game is locked at that resolution.

The game plays perfectly well and there's nothing wrong with the rendering, I'd just like to have it larger on my screen.

Technical information:
I'm running Windows 7 64-bit, and these are the relevant Dosbox settings from the .conf file:

fullscreen=false
fulldouble=true
fullresolution=desktop
windowresolution=1600x1200
output=opengl
autolock=true
sensitivity=100

frameskip=0
aspect=true
scaler=hq3x

I've tried changing the aspect setting, using different scalers and output, and nothing makes a difference.

Any assistance is appreciated. If you have any questions, please feel free to ask. Thank you.

[Edited because I didn't proofread my original post, and had the wrong desired resolution there.]

Whoops, that's my error - I want to play in a 1600x1200 window (so 4:3 ratio, not widescreen), but I wrote "1600x900" not just once but twice in my original post. My apologies.