C# help how to increase image size of imagelist

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C#

i have implemented the sliding image using imagelist and label but the image size is limit to 255x255 but the size of image i want to be 400 x 400
how to set that by default it is 16 x 16 i have set up to 255 x 255 but i want to set to 400 x 400 or more

on click of next button the next image displays but every image displays only in 16 x 16 or 255 x 255 but want increase in size of image pls help
this is the code i have implemented

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Drawing; 
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace CG_LEARN.working {
    public partial class applicationofcg : UserControl
    {
        private int indice;

        public applicationofcg()
        {
            InitializeComponent();
            indice = 0;
        }

        private void button2_Click(object sender, EventArgs e)
        {
            indice++;

            if (indice > 7)
               indice = 0;
               label9.ImageIndex = indice;
        }

    }
}

What I have tried:

i have tried to increase the size of image but not getting increase in image pls help
by default is 16 x 16 and extend up to 255 x 255 but i want more then that 400 x 400 or more