ResizePanel

using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;

public class ResizePanel : MonoBehaviour, IPointerDownHandler, IDragHandler {

public Vector2 minSize = new Vector2 (100, 100);
public Vector2 maxSize = new Vector2 (400, 400);

private RectTransform panelRectTransform;
private Vector2 originalLocalPointerPosition;
private Vector2 originalSizeDelta;

void Awake () {
panelRectTransform = transform.parent.GetComponent<RectTransform> ();
}

public void OnPointerDown (PointerEventData data) {
originalSizeDelta = panelRectTransform.sizeDelta;
RectTransformUtility.ScreenPointToLocalPointInRectangle (panelRectTransform, data.position, data.pressEventCamera, out originalLocalPointerPosition);
}

public void OnDrag (PointerEventData data) {
if (panelRectTransform == null)
return;

Vector2 localPointerPosition;
RectTransformUtility.ScreenPointToLocalPointInRectangle (panelRectTransform, data.position, data.pressEventCamera, out localPointerPosition);
Vector3 offsetToOriginal = localPointerPosition - originalLocalPointerPosition;

Vector2 sizeDelta = originalSizeDelta + new Vector2 (offsetToOriginal.x, -offsetToOriginal.y);
sizeDelta = new Vector2 (
Mathf.Clamp (sizeDelta.x, minSize.x, maxSize.x),
Mathf.Clamp (sizeDelta.y, minSize.y, maxSize.y)
);

panelRectTransform.sizeDelta = sizeDelta;
}
}