Try
SystemParameterInfo with SPI_GETWORKAREA as uiAction-Parameter.
Zitat:
Retrieves the size of the work area on the primary display monitor. The work area is the portion of the screen not obscured by the system taskbar or by application desktop toolbars. The pvParam parameter must point to a RECT structure that receives the coordinates of the work area, expressed in physical pixel size. Any DPI virtualization mode of the caller has no effect on this output.
To get the work area of a monitor other than the primary display monitor, call the GetMonitorInfo function.