GetTextExtentPoint32

C# Signature:

[DllImport("gdi32.dll")]
static extern bool GetTextExtentPoint32(IntPtr hdc, string lpString,
   int cbString, out SIZE lpSize);

Tips & Tricks:

int cbString, out SIZE lpSize);

Sample Code:

Size sz = Size.Empty;
    IntPtr hdc = graphics.GetHdc();
    IntPtr oldfnt = GDI.SelectObject(hdc, font.ToHfont());

    GDI.GetTextExtentPoint32(hdc, text, text.Length,out sz);

    // reset old font
    GDI.DeleteObject(GDI.SelectObject(hdc, oldfnt));
    graphics.ReleaseHdc(hdc);
    return sz;

Alternative Managed API:

using System.Windows.Forms; // for TextRenderer

    Size size=TextRenderer.MeasureText(g, str, f, new Size(), TextFormatFlags.NoPadding);

Alternative Managed API:

g - is the Graphics object you want to draw on
    str - the string that should be measured
    f - the font you want to use
    new Size() - just a dummy parameter