The GetWindowRect and GetClientRect functions can be used calculate the size of all the window borders.
Suite101 has a article on resizing a window and the keeping client area at a know size.
Here is their sample code:
void ClientResize(HWND hWnd, int nWidth, int nHeight)
{
RECT rcClient, rcWind;
POINT ptDiff;
GetClientRect(hWnd, &rcClient);
GetWindowRect(hWnd, &rcWind);
ptDiff.x = (rcWind.right - rcWind.left) - rcClient.right;
ptDiff.y = (rcWind.bottom - rcWind.top) - rcClient.bottom;
MoveWindow(hWnd,rcWind.left, rcWind.top, nWidth + ptDiff.x, nHeight + ptDiff.y, TRUE);
}