Based on my (highly limited) tinkering with DirectX, you'll need to hack the CreateWindow parameters to hold the window size you want, and send the correct flags to DirectX during the initialization. The actual flags depend on the method used to set up the rendering target as well as the DirectX version being used; I'd suggest finding out what version of DirectX the game is written for, and then finding a tutorial on fullscreen-vs-windowed mode in that version, then look for API calls that seem similar. Once you locate the actual DX initialization, hacking the flags should be a pretty trivial bit of reversing.