C#

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace CP_CSharp_SizeButton
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left && this.Cursor!=Cursors.Default)
            {
                if (this.Cursor==Cursors.SizeWE)
                    button1.Width =  e.X;
                else 
                    button1.Height = e.Y;
            }
            else
            {
                if (e.X > button1.Width-5 && e.X < button1.Width+5)
                    this.Cursor=Cursors.SizeWE;
                else if (e.Y > button1.Height-5 && e.Y < button1.Height+5)
                    this.Cursor=Cursors.SizeNS;
                else 
                    this.Cursor = Cursors.Default;
            }
        }

        private void Form1_MouseMove(object sender, MouseEventArgs e)
        {
            this.Cursor = Cursors.Default;
        }
    }
}