Can I set the size of a GUI control in centimeters?

Question

Hi,
    I know that I can get the resolution of the monitor, and with that set the display size of a control in a GUI. However, the real size (in centimeters) depends actually on the sizes (in centimeters) of the screen. Can I get that, programatically?
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Answer 1

After writing #include <windows.h> at the beginning of the program, I wrote the following function called by the main.
void GeometriaGUI(){
HDC dc;
double xPixelsPerCentimer, yPixelsPerCentimer,xPixelsPerMilimeter, yPixelsPerMilimeter;;
dc = GetDC (NULL);
xPixelsPerCentimer = GetDeviceCaps (dc, LOGPIXELSX) / 2.54;
yPixelsPerCentimer = GetDeviceCaps (dc, LOGPIXELSY) / 2.54;
xPixelsPerMilimeter= GetDeviceCaps (dc, HORZRES)/GetDeviceCaps (dc, HORZSIZE);
yPixelsPerMilimeter= /*GetDeviceCaps (dc, VERTRES)/GetDeviceCaps (dc, VERTSIZE);
//ReleaseDC (dc);
SetCtrlAttribute (panelHandle, PNL_INSTR_2, ATTR_HEIGHT, (int)(40.0*yPixelsPerMilimeter));
SetCtrlAttribute (panelHandle, PNL_INSTR_2, ATTR_WIDTH, (int)(40.0*yPixelsPerMilimeter));
 First, I eliminated the ReleaseDC line because it caused an error "Type error in argument 1 to `ReleaseDC'; found 'HDC' expected 'HWND'." at compile time.
If I use xPixelsPerCentimeter to set the size of a control in the GUI, and set a value of 4*xPixelsPerCentimeter, I get, after measuring with a ruler, only 3.5 cm.
If I use 40*xPixelsPerMilimeter,  I get a size of 1.8 cm. In both cases, 4 cm are expected.
If I look at the values given the Windows sdk functions, the ones given by the resolution are atribute are fine (1280 x 800). However, the ones given by HORZSIZE and VERTSIZE attributes don't match what I measure physically. The sizes given are larger than the computer itself (a notebook with only 1 monitor).
The idea is that the size of certain controls are constant across any monitor used. But the function, unfortunately, is not giving the expected results.