This example shows, how to use Splitter component. It allows to get your application in good design style. Put one component to the form and set Align property to alLeft. Put Splitter to the form. Put other component and set Align property to AlClient value. Splitter has MinSize property. If the Align property is alLeft, the splitter can't resize the regions to its left any smaller than MinSize pixels.
procedure TForm1.Button1Click(Sender: TObject); begin Splitter1.MinSize:=300; end;