var textDimensions = GUI.skin.label.CalcSize(new GUIContent("text"));
//textDimensions.x contains the width in pixels.

var textDimensions = GUI.skin.label.CalcSize(new GUIContent(thestring));

while (textDimensions.x > somesize) {
    cut off one character
    textDimensions = GUI.skin.label.CalcSize(new GUIContent(truncated string));
}