// Declare some variables in the dialog class to hold the sizes for 
// your dialog (the separate X sizes aren't actually required for this 
// example, I only include them for completeness, and to show that the 
// code can be adapted for dialogs which expand in X or Y).

long m_lDlgXSizeLarge;
long m_lDlgYSizeLarge;
long m_lDlgXSizeSmall;
long m_lDlgYSizeSmall;
BOOL m_bExtrasVisible;

// In the OnInitDialog handler for your dialog, add the following code:

RECT  rectLarge;
RECT  rectDivider;

GetWindowRect (&rectLarge);
GetDlgItem (IDC_DIVIDER) -> GetWindowRect (&rectDivider);

m_lDlgXSizeLarge = (rectLarge.right - rectLarge.left) + 1;
m_lDlgYSizeLarge = (rectLarge.bottom - rectLarge.top) + 1;
m_lDlgXSizeSmall = m_lDlgXSizeLarge;
m_lDlgYSizeSmall = m_lDlgYSizeLarge  - (rectLarge.bottom - rectDivider.top);
      
HideExtraBits (TRUE); // sets initial state of extras to HIDDEN.

// So you can see the code is effectively saving the expanded and 
// unexpanded sizes for the dialog, based upon the coordinates of the 
// divider static, without saving any absolute screen positions.

// Add a button which when pressed, toggles the hide state as follows:

void CMyDlg::OnExtras()
{
   HideExtraBits (m_bExtrasVisible);
}

// And finally, here's the function which does the hiding and showing:

void CMyDlg::HideExtraBits (BOOL bHide)
{
   if (bHide)
   {
      SetWindowPos (NULL,
                    0, 0,
                    m_lDlgXSizeSmall,
                    m_lDlgYSizeSmall,
                    SWP_NOZORDER|SWP_NOMOVE);
      // Add extra code here to change the legend on your
      // show-hide button if wanted.
   }
   else
   {
      SetWindowPos (NULL,       
                    0, 0,
                    m_lDlgXSizeLarge,
                    m_lDlgYSizeLarge,
                    SWP_NOZORDER|SWP_NOMOVE);
      // Add extra code here to change the legend on your
      // show-hide button if wanted.
   }
   m_bExtrasVisible = !bHide;
}