In developer mode but not in customization mode, go to Actions and click Reset Layouts to Base. Then go into customization mode and save it. (Note; it might be that you also need to click “Save Layouts” when doing this)
Note that like @dhewi said, the position on your screen where this is developed and saved is where it places the size/start location of the form.
If you want code, it would look like this:
//Center the form to screen
private void UD01Form_Load(object sender, EventArgs args)
{
UD01Form.StartPosition = FormStartPosition.CenterScreen;
}
//Make the form fixed size and center screen
private void UD01Form_Load(object sender, EventArgs args)
{
UD01Form.StartPosition = FormStartPosition.CenterScreen;
UD01Form.MaximizeBox = false;
UD01Form.MinimizeBox = false;
UD01Form.FormBorderStyle = FormBorderStyle.FixedDialog;
}