I solved it for now by adding "WINDOWSTATE=wsMaximized" in file lr_view.lfm and recompiling component lazReport, so it now always opens maximized when I use Showreport.
But it would be much nicer if lazReport had WindowState as property in the object inspector, so you can decide per report what you want.
CodeTyphon's next release will have this option, would be nice if Lazarus has it too.