You can iterate through the list to find the longest text extent using MeasureString, and then use this as the ComboBox width adding a fudge factor for the drop-down button.

using ( Graphics g = comboBox1.CreateGraphics() )
{
  float maxWidth = 0f;
  foreach ( object o in comboBox1.Items )
  {
    float w = g.MeasureString( o.ToString(), comboBox1.Font ).Width;
    if ( w > maxWidth ) maxWidth = w;
  }
}
comboBox1.Width = (int) maxWidth + 20; // 20 is to take care of button width

Contributed from George Shepherd's Windows Forms FAQ