listBox1.MouseEnter += new System.EventHandler(listBox1_MouseEnter);
listBox1.MouseLeave += new System.EventHandler(listBox1_MouseLeave);
private void listBox1_MouseEnter(object sender, System.EventArgs e)
{
listBox1.Size = new Size(100, 100)
}
private void listBox1_MouseLeave(object sender, System.EventArgs e)
{
listBox1.Size = new Size(100, 100)
}