The window size seems to be dependent on Desktop size, not screen resolution; however the Desktop conforms to resolution. The trick might be to change the Desktop size or start from there.
A while back, a developer named Robert Bresner (his website) made a product called sDesk (downloadable from here) which expanded Windows beyond the screen resolution.
What is SDesk? SDesk is not a
multi-desktop program. It is a program
that creates a single, giant desktop
that extends beyond the visible region
of the monitor. Only a portion of the
SDesk is visible at a time, as defined
by the Windows desktop settings.
The original homepage of the SDesk software is archived here There's no current page on the Internet (that I could find) but this archived version is accessed through the Way Back Machine which caches a lot of the Internet.
The SDesk product is freeware. No source code was included though. You might visit his contact page (also available in the archived version) to ask if the source is available or can be acquired for your research.