1.    Screen resolution: 800*600 , 96DPI (100%)

Work Area Size as follow:

800*560 (subtract bottom/top taskbar height)

686*600 (subtract left/Right  taskbar height)

2.    Screen resolution: 1024*768, 120DPI (125%)

Get Work Area Size  from  System.Windows.Forms.Screen.PrimaryScreen.Bounds.Size :

1024*720(subtract bottom/top taskbar height)

933*768(subtract  left/Right  taskbar height)

Get Work Area Size from  SystemParameters.WorkArea.Size:

820.0*576.8(subtract bottom/top taskbar height)

747.2*615.2(subtract  left/Right  taskbar height)

This size has divided by DPI(125%).

3.       Screen resolution:1280*800, 144DPI (150%)

Get Work Area Size from  System.Windows.Forms.Screen.PrimaryScreen.Bounds.Size :

1280*744 (subtract bottom/top taskbar height)

1189*800 (subtract  left/Right  taskbar height)

Get Work Area Size from  SystemParameters.WorkArea.Size:

854.0*496.7(subtract bottom/top taskbar height)

793.3*534.0(subtract  left/Right  taskbar height)

This size has divided by DPI(150%).

So , the Min Width is 686, Min Height is 496.7.

Is it Right?