Actually WM_GETMINMAXINFO:
void CInstantMessage::OnGetMinMaxInfo(MINMAXINFO FAR* lpMMI)
{
CDialog::OnGetMinMaxInfo(lpMMI);
RECT winrect;
GetDesktopWindow()->GetClientRect(&winrect);
lpMMI->ptReserved = CPoint(0,0);
lpMMI->ptMaxSize = CPoint(winrect.right,winrect.bottom);
lpMMI->ptMaxPosition = CPoint(0,0);
lpMMI->ptMaxTrackSize = CPoint(winrect.right,winrect.bottom);
lpMMI->ptMinTrackSize = CPoint(235,250);
}
That seems to prevent the user from sizing it smaller than (235,250)