こんな感じか

var startPos : Vector2;
var lastPos : Vector2;
var isDrag : boolean = false;

function OnGUI () {
var e : Event = Event.current;
if (e.type == EventType.MouseDown && e.button == 0) {
lastPos = startPos = e.mousePosition;
isDrag = true;
}
else if (e.type == EventType.MouseUp && e.button == 0) {
isDrag = false;
}
else if (e.type == EventType.MouseDrag) {
lastPos = e.mousePosition;
}
if (isDrag) {
var size = lastPos - startPos;
GUI.Box(new Rect(startPos.x, startPos.y, size.x, size.y), "");
}
}