Set slide/page format dropdown size to 6 in Slide/Page panel

Change-Id: Iad70e5e946c3209246567b7b6449100a49cb8a40
Reviewed-on: https://gerrit.libreoffice.org/28013Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

svx/source/dialog/papersizelistbox.cxx

@@ -27,6 +27,7 @@
 PaperSizeListBox::PaperSizeListBox(vcl::Window* pParent, WinBits nStyle)
     : ListBox( pParent, nStyle)
 {
+    SetDropDownLineCount(6);
 }
 
 VCL_BUILDER_FACTORY(PaperSizeListBox);