If you want to set your form to be a certain size when you run the application, you set the property WindowState in the properties to the size you want : Minimised, Normal, or Maximised.
You can also set this at runtime wherever you choose. The code would look like this:
Me.WindowState = FormWindowState.Maximized
or if you were calling this routine from somewhere other than the form itself
form.WindowState = FormWindowState.Maximized
The form being the name of the form you are trying to change the size of.