To clarify this question, and help you, we need to know:
1. your screen resolution.
2. why you are not focusing on re-sizing the controls to fit your available screen space by using properties such as 'Anchor and 'Dock
The observation that setting the size (and/or maximum size) property of a WinForm larger than the current screen resolution will be ignored, and the current screen size available will be used is correct. This "auto-limit" of Form size will take place both at Design-Time ... and at Run-Time, if you set the Size in code.
OriginalGriff has pointed out to you that one way to get more pixels is to change your screen resolution to a higher resolution.
But do keep in mind that
you can create objects inside Forms that are larger than the maximum Form size, or screen size.
For example, my maximum screen-size right now is about 1300x788, but I can create a Panel 3000x2000, add it to a WinForm, make sure it has no Anchor or Dock property set that will bind it to the Form boundaries; I can position this giant Panel partially or fully off the screen.
And I can write code to click and drag that panel around so that the Form is functioning as a "viewport" on that Panel.
... edit #1 ... you can do the same thing with a UserControl:
UserControl1 uc1 = new UserControl1();
uc1.Dock = DockStyle.None;
uc1.Anchor = AnchorStyles.None;
uc1.MaximumSize = new Size(1500, 1200);
uc1.MinimumSize = new Size(1500, 1200);
uc1.Size = new Size(1500, 1200);
uc1.Location = new Point(-1000, -900);
this.Controls.Add(uc1);
uc1.Show();
Whether Panels or UserControls would meet your needs in this scenario: I don't know.
In any case it would be good to know more about these controls of yours that are "too big:" what are they : can they be put in some container that will let you scroll them ? And so forth.