How to resize a GroupItem's height.

  app.executeMenuCommand('selectall');

  app.executeMenuCommand('group');

  var finishedLayout=app.activeDocument.selection

  finishedLayout.height=NewHeight

finishedLayout.height=NewHeight gives a correct nominal figure in the Data Browser

but on the (CS6 Illustrator) page and inside the GroupItem's data it hasn't changed.

Clearly the methods I've tried so far are the wrong ones - including

finishedLayout.GroupItem.height=NewHeight which gives an error.

Please would someone tell me the correct syntax to use:

sorry - as I've said before, I'm new to this and the documentation

isn't exactly full of illustrations and helpful hints, though this forum is,

and thanks in anticipation of your replies.