To determine the size of the available screen estate in Delphi 3 or earlier, you need to know whether the taskbar is visible or not.
Here's a simple function to check for this:
//------------------------------------------
// Returns TRUE if taskbar auto hide is on.
//
// if IsTaskBarautoHideOn then
// begin
// .. auto hide is ON ..
// end;
//------------------------------------------
function IsTaskbarAutoHideOn : Boolean;
var
ABData : TAppBarData;
begin
ABData.cbSize := SizeOf (ABData);
result := (SHAppBarMessage (ABM_GETSTATE, ABData) and ABS_AUTOHIDE) > 0
end;