You cannot find
Shrink to fit cell size because it is not a menu item.
Format Cells is a menu item which opens the dialog where
Alignment tab contains
Shrink to fit cell size.
Edit: Another alternative is to write your own macro and then create a toolbar item to execute the macro.
This routine is a toggle… When run, if normal it will Shrink selected cells and if Shrink is on it will turn off.
Dim oCurrSel As Object
oCurrSel = ThisComponent.getCurrentSelection()
If oCurrSel.ShrinkToFit Then
oCurrSel.ShrinkToFit = False
oCurrSel.ShrinkToFit = True