Inno Setup: how can I resize the lower pane area?

The lower pane are is actually an empty area. So you just need to increase the window height and decrease height of the OuterNotebook and shift up the Bevel:

[Code]

procedure IncreaseFooter(Delta: Integer);
begin
  WizardForm.Height := WizardForm.Height + Delta;
  WizardForm.OuterNotebook.Height := WizardForm.OuterNotebook.Height - Delta;
  WizardForm.Bevel.Top := WizardForm.Bevel.Top - Delta;
end;

procedure InitializeWizard();
begin
  IncreaseFooter(ScaleY(46));
end;