Look in the xorg.conf man page under "Display subsection". I think you may want to add a Virtual entry.
Virtual xdim ydim
This optional entry specifies the virtual screen resolution to be used. xdim must be a multiple of either 8 or 16 for most drivers,
and a multiple of 32 when running in monochrome mode. The given value will be rounded down if this is not the case. Video modes
which are too large for the specified virtual size will be rejected. If this entry is not present, the virtual screen resolution
will be set to accommodate all the valid video modes given in the Modes entry. Some drivers/hardware combinations do not support
virtual screens. Refer to the appropriate driver-specific documentation for details.
That would give you a virtual screen larger than your real screen.
Another option for reaching the bottom of windows and dialogs, such as clicking an "OK" box hidden below the screen, is to hold down the [ALT] key, press anywhere on the window or dialog box and drag it, to reach the part you need. This won't work for drop down menu's but will for moving windows.