Use the AutoSize and AutoSizeMode properties.
http://msdn.microsoft.com/en-us/library/system.windows.forms.form.autosize.aspx
An example:
private void Form1_Load(object sender, EventArgs e)
{
// no smaller than design time size
this.MinimumSize = new System.Drawing.Size(this.Width, this.Height);
// no larger than screen size
this.MaximumSize = new System.Drawing.Size(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height, (int)System.Windows.SystemParameters.PrimaryScreenHeight);
this.AutoSize = true;
this.AutoSizeMode = AutoSizeMode.GrowAndShrink;
// rest of your code here...
}