window size in Windows User Experience Interaction Guidelines

  Support the minimum Windows effective resolution of 800x600 pixels. For critical user interfaces (UIs) that
must work in safe mode, support an effective resolution of 640x480 pixels. Be sure to account for the space
used by the taskbar by reserving 48 vertical relative pixels for windows displayed with the taskbar.
  Optimize resizable window layouts for an effective resolution of 1024x768 pixels. Automatically resize these
windows for lower screen resolutions in a way that is still functional.
  Be sure to test your windows in 96 dots per inch (dpi) (at 800x600 pixels), 120 dpi (at 1024x768 pixels), and
144 dpi (at 1200x900 pixels) modes. Check for layout problems, such as clipping of controls, text, and
windows, and stretching of icons and bitmaps.
  For programs with touch and mobile use scenarios, optimize for 120 dpi. High-dpi screens are currently
prevalent on touch and mobile PCs.
  If a window is an owned window, initially display it "centered" on top of the owner window. Never
underneath. For subsequent display, consider displaying it in its last location (relative to the owner window) if
doing so is likely to be more convenient.
  If a window is contextual, always display it near the object that it was launched from. However, place it out
of the way (preferably offset down and to the right) so that the object isn't covered by the window.