In the original game, in 640x480 the dialog box also took almost half of the screen. Indeed in 856x480 it's a little higher than that, but unless something major happens, I won't be updating this mod again.
The mod can accept any custom resolution, so if you add a custom widescreen resolution slightly higher than 856x480 --either through your GPU's panel or by editing the registry-- maybe you'll get what you want.