// Draws a window you can resize between 80px and 200px width
// Just click the box inside the window and move your mouse
private var windowRect : Rect = Rect (10, 10, 100, 100);
private var scaling : boolean = false;

function OnGUI () {
    windowRect = GUILayout.Window (0, windowRect, ScalingWindow, "resizeable", 
                       GUILayout.MinWidth(80), GUILayout.MaxWidth(200));
}

function ScalingWindow (windowID : int) {
    GUILayout.Box("", GUILayout.Width(20), GUILayout.Height(20));
    if (Event.current.type == EventType.MouseUp) {
        scaling = false;
    }
    else if (Event.current.type == EventType.MouseDown && 
         GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition)) {
        scaling = true;
    }
    
    if (scaling) {
        windowRect = Rect (windowRect.x, windowRect.y, 
               windowRect.width + Event.current.delta.x, windowRect.height + Event.current.delta.y);
    }
}

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    private Rect windowRect = new Rect(10, 10, 100, 100);
    private bool scaling = false;
    void OnGUI() {
        windowRect = GUILayout.Window(0, windowRect, ScalingWindow, "resizeable", GUILayout.MinWidth(80), GUILayout.MaxWidth(200));
    }
    void ScalingWindow(int windowID) {
        GUILayout.Box("", GUILayout.Width(20), GUILayout.Height(20));
        if (Event.current.type == EventType.MouseUp)
            scaling = false;
        else
            if (Event.current.type == EventType.MouseDown && GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition))
                scaling = true;
            
        if (scaling)
            windowRect = new Rect(windowRect.x, windowRect.y, windowRect.width + Event.current.delta.x, windowRect.height + Event.current.delta.y);
        
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    private windowRect as Rect = Rect(10, 10, 100, 100)

    private scaling as bool = false

    def OnGUI():
        windowRect = GUILayout.Window(0, windowRect, ScalingWindow, 'resizeable', GUILayout.MinWidth(80), GUILayout.MaxWidth(200))

    def ScalingWindow(windowID as int):
        GUILayout.Box('', GUILayout.Width(20), GUILayout.Height(20))
        if Event.current.type == EventType.MouseUp:
            scaling = false
        elif (Event.current.type == EventType.MouseDown) and GUILayoutUtility.GetLastRect().Contains(Event.current.mousePosition):
            scaling = true
        if scaling:
            windowRect = Rect(windowRect.x, windowRect.y, (windowRect.width + Event.current.delta.x), (windowRect.height + Event.current.delta.y))