It depends on the rendering engine being used. You can basically switch between GDI and GDI+. Switching can be done by setting the UseCompatibleTextRendering property accordingly
When using GDI+ you should use MeasureString:
string s = "A sample string";
SizeF size = e.Graphics.MeasureString(s, new Font("Arial", 24));
When using GDI (i.e. the native Win32 rendering) you should use the TextRenderer class:
SizeF size = TextRenderer.MeasureText(s, new Font("Arial", 24));
See this article: Text Rendering: Build World-Ready Apps Using Complex Scripts In Windows Forms Controls