定义变量存储原始尺寸

private int origin_width = 0;
private int origin_height = 0;

获取图片的原始尺寸

this.origin_width = this.pictureBox1.Width;
this.origin_height = this.pictureBox1.Height;

关联双击事件

this.pictureBox1.DoubleClick += pictureBox1_DoubleClick;

处理双击事件

//双击图片时进入全屏或者还原
void pictureBox1_DoubleClick(object sender, EventArgs e)
{
	if (this.WindowState == FormWindowState.Maximized)
	{
		this.FormBorderStyle = FormBorderStyle.Sizable;
		this.WindowState = FormWindowState.Normal;
		this.pictureBox1.Width = this.origin_width;
		this.pictureBox1.Height = this.origin_height;
		this.pictureBox1.Dock = DockStyle.None;
		this.pictureBox1.Refresh();
		this.Refresh();
	}
	else
	{
		this.FormBorderStyle = FormBorderStyle.None;
		this.WindowState = FormWindowState.Maximized;
		this.pictureBox1.Dock = DockStyle.Fill;
		this.pictureBox1.Refresh();
		this.Refresh();
	}
}