It would be like this:
procedure TForm1.DBAdvGrid1CustomCellDraw(Sender: TObject; Canvas: TCanvas;
ACol, ARow: Integer; AState: TGridDrawState; ARect: TRect; Printing: Boolean);
if ARow > 0 then
DBAdvGrid1.RowHeights[ARow] := 120;
But do not modify row heights from a drawing event. Such event is triggered frequently, and is used exclusively for content painting, not for adjusting content size. What's worse, if you e.g. allowed row sizing and the user would try to setup row height, it would in turn trigger that event where you would change the height back, so you'd be fighting with the user.
Content sizing should be done earlier, as this example shows in the OnCustomCellSize event.
But for your aim I think it's enough to set DefaultRowHeight and FixedRowHeight properties with no additional code.