You can call all the methods which the toolbar button would execute directly in TWPRichText.
I recommend to check out the method OnToolBarIconSelection in WPCtrRich.pas
You will find code like this:
Code
WPI_CO_FitWidth:
begin
if AutoZoom <> wpAutoZoomWidth then
AutoZoom := wpAutoZoomWidth
else
AutoZoom := wpAutoZoomOff;
Typ := wptNone;
DoUpdateParAttr;
end;
Which is the code executed by the "FitWidth" Icon.