GettextExtentExPoint

[DllImport("coredll.dll", SetLastError = true, CharSet = CharSet.Unicode)]
public static extern bool GettextExtentExPoint(
   IntPtr hdc,
   string lpszString,
   int cchString,
   int nMaxExtent,
   out int lpnFit,
   out int[] alpDx,
   out SIZE lpSize
);

Microsoft documentation: Link